mbuf: add accessor function for private data area

mbuf: add accessor function for private data area

Add an inline accessor function to return the starting address of
the private data area in the supplied mbuf.

This allows applications to easily access the private data area between
the struct rte_mbuf and the data buffer in the specified mbuf without
creating private macros or accessor functions.

No checks are made to ensure that a private data area actually exists
in the buffer.

Signed-off-by: Dan Gora <dg@adax.com>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>