sdp: device_removal_lock should not be a spinlock because module removal takes a...

sdp: device_removal_lock should not be a spinlock because module removal takes a long time.

In order to prevent a situation that many CPUs are stack with read-spinlock
waiting for remove-device write-lock to be freed, I replaced the rw-spinlock
with rw-semaphore. That way the blocked processes goes to sleep and are not
blocking other processes.

Signed-off-by: Eldad Zinger <eldadz@mellanox.co.il>