commit 3b785fbc upstream.

This avoids that new requests are queued while __dm_destroy() is in
progress.
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
[js: use md->queue instead of non-present helper]
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>