scsi/scsi_tgt_lib: scsi_tgtd isn't used in memory reclaim path

Workqueue scsi_tgtd isn't used during memory reclaim.  Convert to
alloc_workqueue() without WQ_MEM_RECLAIM.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: FUJITA Tomonori <tomof@acm.org>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: linux-scsi@vger.kernel.org