scx_flatcg: Make good use of __sync_fetch_and_sub()

Fetch the value of "delta" directly from the returned value from
__sync_fetch_and_sub, as it returns the origin value of
cgc->cvtime_delta.

Addional fetching instruction of cgc->cvtime_delta would be redundant
here.

Signed-off-by: I Hsin Cheng <[email protected]>

scheds/c/scx_flatcg.bpf.c

@@ -258,8 +258,7 @@ static void cgrp_cap_budget(struct cgv_node *cgv_node, struct fcg_cgrp_ctx *cgc)
 	 * and thus can't be updated and repositioned. Instead, we collect the
 	 * vtime deltas separately and apply it asynchronously here.
 	 */
-	delta = cgc->cvtime_delta;
-	__sync_fetch_and_sub(&cgc->cvtime_delta, delta);
+	delta = __sync_fetch_and_sub(&cgc->cvtime_delta, delta);
 	cvtime = cgv_node->cvtime + delta;
 
 	/*