scsi: ufs: Add clock ungating to a separate workqueue

UFS driver can receive a request during memory reclaim by kswapd.
So when ufs driver puts the ungate work in queue, and if there are no
idle workers, kthreadd is invoked to create a new kworker. Since
kswapd task holds a mutex which kthreadd also needs, this can cause
a deadlock situation. So ungate work must be done in a separate
work queue with WQ__RECLAIM flag enabled.Such a workqueue will have
a rescue thread which will be called when the above deadlock
condition is possible.

Change-Id: I91c5bff812be403addc98fbb8c620e67cf28f5d9
Signed-off-by: Vijay Viswanath <vviswana@codeaurora.org>
[cang@codeaurora.org: resolved trivial merge conflicts]
Signed-off-by: Can Guo <cang@codeaurora.org>