net/mlx4: When issuing commands use rwsem insteam of rw spinlocks

net/mlx4: When issuing commands use rwsem insteam of rw spinlocks

The mlx4 drivers use a read_lock while issuing commands, but
when a lot of commands are issued simultaneously, the mlx4
driver could sleep. In order to resolve this "sleep while holding
spin-lock" issue, we replace this spinlock with read-write
semaphore.

Fixes: 2393fac27a97 ('net/mlx4: Switching between sending
		     commands via polling and events may results
		     in hung tasks')
Signed-off-by: Matan Barak <matanb@mellanox.com>
(Ported from Mellanox OFED 2.4)

Signed-off-by: Mukesh Kacker <mukesh.kacker@oracle.com>