IB/mlx4: Lock SQ lock in mlx4_ib_post_send()

Because of a typo, mlx4_ib_post_send() takes the same lock rq.lock as
mlx4_ib_post_recv().  Correct the code so the intended sq.lock is
taken when posting a send.

Noticed by Yossi Leybovitch and pointed out by Jack Morgenstein from
Mellanox.

Signed-off-by: Roland Dreier <rolandd@cisco.com>