mm/damon/sysfs: avoid call damon_target_has_pid() repeatedly

In damon_sysfs_destroy_targets(), we call damon_target_has_pid() to check
whether the 'ctx' include a valid pid, but there no need to call
damon_target_has_pid() to check repeatedly, just need call it once.

[xhao@linux.alibaba.com: more simplified code calls damon_target_has_pid()]
  Link: https://lkml.kernel.org/r/20220916133535.7428-1-xhao@linux.alibaba.com
Link: https://lkml.kernel.org/r/20220915142237.92529-1-xhao@linux.alibaba.comSigned-off-by: NXin Hao <xhao@linux.alibaba.com>
Reviewed-by: NSeongJae Park <sj@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>

mm/damon/sysfs.c

@@ -2143,9 +2143,10 @@ static int damon_sysfs_set_attrs(struct damon_ctx *ctx,
 static void damon_sysfs_destroy_targets(struct damon_ctx *ctx)
 {
 	struct damon_target *t, *next;
+	bool has_pid = damon_target_has_pid(ctx);
 
 	damon_for_each_target_safe(t, next, ctx) {
-		if (damon_target_has_pid(ctx))
+		if (has_pid)
 			put_pid(t->pid);
 		damon_destroy_target(t);
 	}