mbuf: support attaching external buffer

mbuf: support attaching external buffer

This patch introduces a new way of attaching an external buffer to a mbuf.

Attaching an external buffer is quite similar to mbuf indirection in
replacing buffer addresses and length of a mbuf, but a few differences:
  - When an indirect mbuf is attached, refcnt of the direct mbuf would be
    2 as long as the direct mbuf itself isn't freed after the attachment.
    In such cases, the buffer area of a direct mbuf must be read-only. But
    external buffer has its own refcnt and it starts from 1. Unless
    multiple mbufs are attached to a mbuf having an external buffer, the
    external buffer is writable.
  - There's no need to allocate buffer from a mempool. Any buffer can be
    attached with appropriate free callback.
  - Smaller metadata is required to maintain shared data such as refcnt.

Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>