Bitmap lock/unlock were added to bdrv_enable_dirty_bitmap in
8b1402ce, but some places were not updated correspondingly, which
leads to trying to take this lock twice, which is dead-lock. Fix this.

Actually, iotest 199 (about dirty bitmap postcopy migration) is broken
now, and this fixes it.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20180625165745.25259-3-vsementsov@virtuozzo.com
Signed-off-by: NJohn Snow <jsnow@redhat.com>