net/mlx5: add reference counter on memory region

net/mlx5: add reference counter on memory region

This patch introduce the Memory region as a shared object where users
should get a reference to it by calling the priv_mr_get() or
priv_mr_new() to create the memory region.  This last one will
register the memory pool in the kernel driver and retrieve the
associated memory region.

This should help to reduce the memory consumption cause by registering
multiple times the same memory pool.

Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>