mbuf: move definitions into a separate file

mbuf: move definitions into a separate file

Right now inclusion of rte_mbuf.h header can cause inclusion of
some arch/os specific headers.
That prevents it to be included directly by some
non-DPDK (but related) entities: KNI, BPF programs, etc.
To overcome that problem usually a separate definitions of rte_mbuf
structure is created within these entities.
That aproach has a lot of drawbacks: code duplication, error prone, etc.
This patch moves rte_mbuf structure definition (and some related macros)
into a separate file that can be included by both rte_mbuf.h and
other non-DPDK entities.

Note that it doesn't introduce any change for current DPDK code.

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Michel Machado <michel@digirati.com.br>
Acked-by: Olivier Matz <olivier.matz@6wind.com>