gnrc_pktbuf_static: fix alignment issue / leaks
gnrc_pktbuf_static: fix alignment issue / leaks

This fixes an alignment issue I encountered in the static version of
the packet buffer.

The bug is caused by a race-condition where a certain order of
operations leads to a chunk being released according to the
byte-alignment of the platform, but overlapping potential space for
a future `_unused_t` struct e.g. (x mark allocated regions):

                    Future leak of size sizeof(_unused_t)       Time
                    v                                            |
    +------------+-----+--------------------+                    |
    |xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx|                    +
    +------------+-----+--------------------+                    |
                                                                 |
    +------------+--+--+--------------------+                    |
    |               |xxxxxxxxxxxxxxxxxxxxxxx|                    +
    +------------+--+--+--------------------+                    |
                                                                 |
    +-----+------+--+--+--------------------+                    |
    |xxxxx|         |xxxxxxxxxxxxxxxxxxxxxxx|                    +
    +-----+------+--+--+--------------------+                    |
                                                                 |
    +-----+------+-----+---------+----------+                    |
    |xxxxx|                      |xxxxxxxxxx|                    +
    +-----+------+-----+---------+----------+                    |
                                                                 |
    +-----+------+-----+--------------------+                    |
    |xxxxx|      |xxxxxxxxxxxxxxxxxxxxxxxxxx|                    +
    +-----+------+-----+--------------------+                    |
                                                                 |
    +------------+-----+--------------------+                    |
    |xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx|                    +
    +------------+-----+--------------------+                    |
                                                                 |
    +------------+-----+--------------------+                    |
    |xxxxxxxxxxxxxxxxxx|                    |                    +
    +------------+-----+--------------------+                    |
                                                                 |
    +------------+-----+--------------------+                    |
    |            |xxxxx|                    |                    +
    +------------+-----+--------------------+                    |
                                                                 v

Sadly, I wasn't able to create a reproducable unittest that show-cases
this corner-case, since I don't understand the order of operations that
cause this one 100%, but the bug is reproducable (but also not
reliably) by sending large (i.e. fragmented) packets to a 6Lo-enabled
host from more than 1 host simultaneously (use `gnrc_pktbuf_cmd` to
check).

By making the size of `_unused_t` the only condition for alignment,
this bug is fixed.

BytesGalore/RIOT,gebart/RIOT,mtausig/RIOT,A-Paul/RIOT,smlng/RIOT,cladmi/RIOT,cladmi/RIOT,basilfx/RIOT,rfuentess/RIOT,yogo1212/RIOT,miri64/RIOT,x3ro/RIOT,smlng/RIOT,rfuentess/RIOT,smlng/RIOT,yogo1212/RIOT,BytesGalore/RIOT,biboc/RIOT,biboc/RIOT,ant9000/RIOT,ant9000/RIOT,authmillenon/RIOT,mtausig/RIOT,jasonatran/RIOT,kYc0o/RIOT,kbumsik/RIOT,avmelnikoff/RIOT,BytesGalore/RIOT,yogo1212/RIOT,miri64/RIOT,ant9000/RIOT,yogo1212/RIOT,authmillenon/RIOT,kaspar030/RIOT,kbumsik/RIOT,gebart/RIOT,OTAkeys/RIOT,miri64/RIOT,yogo1212/RIOT,jasonatran/RIOT,gebart/RIOT,miri64/RIOT,kbumsik/RIOT,ant9000/RIOT,biboc/RIOT,aeneby/RIOT,authmillenon/RIOT,kbumsik/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,jasonatran/RIOT,x3ro/RIOT,x3ro/RIOT,kYc0o/RIOT,cladmi/RIOT,lazytech-org/RIOT,gebart/RIOT,mfrey/RIOT,OlegHahm/RIOT,basilfx/RIOT,cladmi/RIOT,avmelnikoff/RIOT,x3ro/RIOT,kYc0o/RIOT,miri64/RIOT,avmelnikoff/RIOT,ant9000/RIOT,rfuentess/RIOT,toonst/RIOT,toonst/RIOT,toonst/RIOT,josephnoir/RIOT,A-Paul/RIOT,kaspar030/RIOT,OlegHahm/RIOT,rfuentess/RIOT,aeneby/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kbumsik/RIOT,OTAkeys/RIOT,lazytech-org/RIOT,A-Paul/RIOT,kaspar030/RIOT,kYc0o/RIOT,authmillenon/RIOT,aeneby/RIOT,avmelnikoff/RIOT,josephnoir/RIOT,A-Paul/RIOT,basilfx/RIOT,BytesGalore/RIOT,mfrey/RIOT,kYc0o/RIOT,smlng/RIOT,josephnoir/RIOT,OlegHahm/RIOT,mfrey/RIOT,biboc/RIOT,RIOT-OS/RIOT,mfrey/RIOT,avmelnikoff/RIOT,mfrey/RIOT,mtausig/RIOT,authmillenon/RIOT,mtausig/RIOT,kaspar030/RIOT,A-Paul/RIOT,lazytech-org/RIOT,authmillenon/RIOT,BytesGalore/RIOT,gebart/RIOT,OTAkeys/RIOT,mtausig/RIOT,biboc/RIOT,basilfx/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,toonst/RIOT,jasonatran/RIOT,lazytech-org/RIOT,x3ro/RIOT,aeneby/RIOT,smlng/RIOT,yogo1212/RIOT,basilfx/RIOT,OTAkeys/RIOT,rfuentess/RIOT,lazytech-org/RIOT,toonst/RIOT,RIOT-OS/RIOT,cladmi/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,aeneby/RIOT,OlegHahm/RIOT,kaspar030/RIOT