migration/block-dirty-bitmap: fix dirty_bitmap_mig_before_vm_start

migration/block-dirty-bitmap: fix dirty_bitmap_mig_before_vm_start

Using the _locked version of bdrv_enable_dirty_bitmap to bypass locking
is wrong as we do not already own the mutex.  Moreover, the adjacent
call to bdrv_dirty_bitmap_enable_successor grabs the mutex.

Fixes: 58f72b965e9e1q
Cc: qemu-stable@nongnu.org # v3.0
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200727194236.19551-8-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit e6ce5e92248be5547daaee3eb6cd226e9820cf7b)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>