IB/mlx5: Fix cached MR allocation flow

When we have a miss in one order of the mkey cache, we try to get
an mkey from a higher order.

We still need to check that the higher order can be used with UMR
before using it. Otherwise, we will get an mkey with 0 entries and
the post send operation that is used to fill it will complete with
the following error:

mlx5_0:dump_cqe:275:(pid 0): dump error cqe
00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000
00000000 0f007806 25000025 49ce59d2

Fixes: 49780d42 ("IB/mlx5: Expose MR cache for mlx5_ib")
Cc: <stable@vger.kernel.org> # v4.10+
Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Reviewed-by: Ilya Lesokhin <ilyal@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>