blk-mq: skip CPU offline notify on unmapped hctx

blk-mq: skip CPU offline notify on unmapped hctx

[ Upstream commit 10845a105bbcb030647a729f1716c2309da71d33 ]

If an hctx has no software ctx mapped, blk_mq_map_swqueue() never
allocates tags and leaves hctx->tags NULL. The CPU hotplug offline
notifier can still run for that hctx, return early since hctx cannot
hold any requests.

Signed-off-by: Cong Zhang <cong.zhang@oss.qualcomm.com>
Fixes: bf0beec0607d ("blk-mq: drain I/O when all CPUs in a hctx are offline")
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>