block: Remove NULL check in bdrv_co_flush

Reported by Coverity. We already use bs in bdrv_inc_in_flight before
checking for NULL. It is unnecessary as all callers pass non-NULL bs, so
drop it.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/io.c

@@ -2300,7 +2300,7 @@ int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
 
     bdrv_inc_in_flight(bs);
 
-    if (!bs || !bdrv_is_inserted(bs) || bdrv_is_read_only(bs) ||
+    if (!bdrv_is_inserted(bs) || bdrv_is_read_only(bs) ||
         bdrv_is_sg(bs)) {
         goto early_exit;
     }