mbuf: avoid cast-align warning in data offset macro
mbuf: avoid cast-align warning in data offset macro

In rte_pktmbuf_mtod_offset macro, there is a casting from char * to type
't', which may cause cast-align warning when using strict cast align
flag with supporting gcc:
gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
CFLAGS="-Wcast-align=strict" make V=1 -C examples/l2fwd clean static

main.c: In function 'l2fwd_mac_updating':
/dpdk/build/include/rte_mbuf_core.h:719:3: warning: cast increases
required alignment of target type [-Wcast-align]
  719 |  ((t)((char *)(m)->buf_addr + (m)->data_off + (o)))
      |   ^
/dpdk/build/include/rte_mbuf_core.h:733:32: note: in expansion of macro
'rte_pktmbuf_mtod_offset'
  733 | #define rte_pktmbuf_mtod(m, t) rte_pktmbuf_mtod_offset(m, t, 0)
      |                                ^~~~~~~~~~~~~~~~~~~~~~~

As the code assumes correct alignment, add first a (void *) casting, to
avoid the warning.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org

Signed-off-by: Eli Britstein <c54ad588c3167261842bc40039d7bd4571322ce1@nvidia.com>
Acked-by: Olivier Matz <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>

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