IB/mlx5: Fix binary compatibility with libmlx5

Commit c1be5232 ("Fix micro UAR allocator") broke binary compatibility
between libmlx5 and mlx5_ib since it defines a different value to the number
of micro UARs per page, leading to wrong calculation in libmlx5. This patch
defines struct mlx5_ib_alloc_ucontext_req_v2 as an extension to struct
mlx5_ib_alloc_ucontext_req.  The extended size is determined in mlx5_ib_alloc_ucontext()
and in case of old library we use uuarn 0 which works fine -- this is
acheived due to create_user_qp() falling back from high to medium then to
low class where low class will return 0.  For new libraries we use the
more sophisticated allocation algorithm.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>