meminfo: fix report swap usage
meminfo: fix report swap usage

when swap controller is enabled but no limits are set, the limits
show up as (unsigned) -1.  This messes up, for instance, 'free -m'.

Closes #56.

Signed-off-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

evgeni/pkg-lxcfs-debian,Xeno22/lxcfs,Xeno22/lxcfs,evgeni/pkg-lxcfs-debian