hash: silence warning on pointer arithmetic

hash: silence warning on pointer arithmetic

Since sig_tbl_bucket_size and key_tbl_key_size are explicitly aligned
at initialization, offset dereferences in the hash table code cannot
possibly be unaligned.  However, the compiler is unaware of this fact
and complains on -Wcast-align.  This patch modifies the code to use
RTE_PTR_ADD(), thereby silencing the compiler by casting through (void
*).

Signed-off-by: Cyril Chemparathy <cchemparathy@ezchip.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>