block/dirty-bitmaps: fix merge permissions

block/dirty-bitmaps: fix merge permissions

In prior commits that made merge transactionable, we removed the
assertion that merge cannot operate on disabled bitmaps. In addition,
we want to make sure that we are prohibiting merges to "locked" bitmaps.

Use the new user_locked function to check.

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20181002230218.13949-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>