RDMA/mlx5: Fix multiple NULL-ptr deref errors in rereg_mr flow

Failure in rereg MR releases UMEM but leaves the MR to be destroyed
by the user. As a result the following scenario may happen:
"create MR -> rereg MR with failure -> call to rereg MR again" and
hit "NULL-ptr deref or user memory access" errors.

Ensure that rereg MR is only performed on a non-dead MR.

Cc: syzkaller <syzkaller@googlegroups.com>
Cc: <stable@vger.kernel.org> # 4.5
Fixes: 395a8e4c

 ("IB/mlx5: Refactoring register MR code")
Reported-by: Noa Osherovich <noaos@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>