scsi/libiscsi: Fix static checker warning on bh locking

scsi/libiscsi: Fix static checker warning on bh locking

Commit 659743b "[SCSI] libiscsi: Reduce locking contention in fast path" introduced a
new smatch warning on libiscsi.c "iscsi_xmit_task() warn: inconsistent returns
bottom_half:: locked (1410 [(-61)]) unlocked (1425 [0], 1425 [s32min-(-1),1-s32max])",
which we can eliminate by using non bh locking on the nested spin_lock call.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Shlomo Pongratz <shlomop@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Christoph Hellwig <hch@lst.de>