block: Use bdrv_reopen_set_read_only() in external_snapshot_commit()

This patch replaces the bdrv_reopen() call that set and remove the
BDRV_O_RDWR flag with the new bdrv_reopen_set_read_only() function.
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -1703,8 +1703,7 @@ static void external_snapshot_commit(BlkActionState *common)
      * bdrv_reopen_multiple() across all the entries at once, because we
      * don't want to abort all of them if one of them fails the reopen */
     if (!atomic_read(&state->old_bs->copy_on_read)) {
-        bdrv_reopen(state->old_bs, state->old_bs->open_flags & ~BDRV_O_RDWR,
-                    NULL);
+        bdrv_reopen_set_read_only(state->old_bs, true, NULL);
     }
 
     aio_context_release(aio_context);