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

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>