IB/mlx5: Allow creation of QPs with zero-length work queues

The current code attmepts to call ib_umem_get() even if the length is
zero, which causes a failure. Since the spec allows zero length work
queues, change the code so we don't call ib_umem_get() in those cases.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>