target: Convert se_lun->lun_deve_lock to normal spinlock

This patch converts se_lun->lun_deve_lock acquire/release access
to use a normal, non bottom-half spin_lock_t for protecting
se_lun->lun_deve_list access.

Reported-by: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>