mbuf: silence warning on pointer arithmetic
mbuf: silence warning on pointer arithmetic

Translating from an mbuf element to the mbuf pointer does not break alignment
constraints.  However, the compiler is unaware of this fact and complains on
-Wcast-align.  This patch modifies the code to use RTE_PTR_SUB(), thereby
silencing the compiler by casting through (void *).

Signed-off-by: Cyril Chemparathy <eff9b25a02e40cc8a3060ecfabdb379b97435a4e@ezchip.com>
Acked-by: Olivier Matz <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>

venkynv/dpdk-mirror,msune/dpdk,tsphillips/dpdk-fork,msune/dpdk,mixja/dpdk,mixja/dpdk,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork,venkynv/dpdk-mirror,venkynv/dpdk-mirror,venkynv/dpdk-mirror,mixja/dpdk,msune/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,mixja/dpdk,msune/dpdk,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork