block/dirty-bitmap: drop BdrvDirtyBitmap.mutex

block/dirty-bitmap: drop BdrvDirtyBitmap.mutex

mutex field is just a pointer to bs->dirty_bitmap_mutex, so no needs
to store it in BdrvDirtyBitmap when we have bs pointer in it (since
previous patch).

Drop mutex field. Constantly use bdrv_dirty_bitmaps_lock/unlock in
block/dirty-bitmap.c to make it more obvious that it's not per-bitmap
lock. Still, for simplicity, leave bdrv_dirty_bitmap_lock/unlock
functions as an external API.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20190916141911.5255-4-vsementsov@virtuozzo.com
Signed-off-by: John Snow <jsnow@redhat.com>