nvme: call blk_drain in NVMe reset code to avoid lockups

nvme: call blk_drain in NVMe reset code to avoid lockups

When blk_flush called in NVMe reset path S/C queues are already freed
which means that re-entering AIO handling loop having some IO requests
unfinished will lockup or crash as their SG structures being potentially
reused. Call blk_drain before freeing the queues to avoid this nasty
scenario.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>