mm: swapfile: check usable swap device in __folio_throttle_swaprate()

Skip blk_cgroup_congested() if there is no usable swap device since no
swapin/out will occur, Thereby avoid taking swap_lock.  The difference
is shown below from perf date of CoW pagefault,

  perf report -g -i perf.data.swapon  | egrep "blk_cgroup_congested|__folio_throttle_swaprate"
      1.01%     0.16%  page_fault2_pro  [kernel.kallsyms]      [k] __folio_throttle_swaprate
      0.83%     0.80%  page_fault2_pro  [kernel.kallsyms]      [k] blk_cgroup_congested

  perf report -g -i perf.data.swapoff   | egrep  "blk_cgroup_congested|__folio_throttle_swaprate"
      0.15%     0.15%  page_fault2_pro  [kernel.kallsyms]      [k] __folio_throttle_swaprate

Link: https://lkml.kernel.org/r/20240418135644.2736748-1-wangkefeng.wang@huawei.comSigned-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/swapfile.c

@@ -2444,13 +2444,17 @@ static void reinsert_swap_info(struct swap_info_struct *p)
 	spin_unlock(&swap_lock);
 }
 
+static bool __has_usable_swap(void)
+{
+	return !plist_head_empty(&swap_active_head);
+}
+
 bool has_usable_swap(void)
 {
-	bool ret = true;
+	bool ret;
 
 	spin_lock(&swap_lock);
-	if (plist_head_empty(&swap_active_head))
-		ret = false;
+	ret = __has_usable_swap();
 	spin_unlock(&swap_lock);
 	return ret;
 }
@@ -3710,6 +3714,9 @@ void __folio_throttle_swaprate(struct folio *folio, gfp_t gfp)
 	if (!(gfp & __GFP_IO))
 		return;
 
+	if (!__has_usable_swap())
+		return;
+
 	if (!blk_cgroup_congested())
 		return;