scsi: ufs: Use WQ_HIGHPRI for gating work.

Must have WQ_MEM_RECLAIM
``WQ_MEM_RECLAIM``
  All wq which might be used in the memory reclaim paths **MUST**
  have this flag set.  The wq is guaranteed to have at least one
  execution context regardless of memory pressure.

Bug: 158050260
Bug: 155410470
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
Link: https://lore.kernel.org/linux-scsi/20200915204532.1672300-3-jaegeuk@kernel.org/T/#u
(cherry picked from commit dd6da262a7af74bd46c8ca93fc305381ee3a3181)
Change-Id: Id2bca586f416a6be07f31ff2982113740cc879a0