mbuf: add macro for offset arithmetic

There are a number of instances in the code where rte_pktmbuf_mtod()
is used to get the mbuf data pointer, only to add an offset before
casting the result to some other header type.  This patch adds a new
rte_pktmbuf_mtod_offset() macro to eliminate these awful double cast
situations.

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