blk-mq: Fix tagset reinit in the presence of cpu hot-unplug

blk-mq: Fix tagset reinit in the presence of cpu hot-unplug

[ Upstream commit 0067d4b020ea07a58540acb2c5fcd3364bf326e0 ]

In case cpu was unplugged, we need to make sure not to assume
that the tags for that cpu are still allocated. so check
for null tags when reinitializing a tagset.

Reported-by: Yi Zhang <yizhan@redhat.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>