f2fs: relocate the tracepoint for background_gc

Once f2fs_gc is done, wait_ms is changed once more.
So, its tracepoint would be located after it.
Reported-by: NHe YunLei <heyunlei@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.c

@@ -77,13 +77,13 @@ static int gc_thread_func(void *data)
 
 		stat_inc_bggc_count(sbi);
 
-		trace_f2fs_background_gc(sbi->sb, wait_ms,
-				prefree_segments(sbi), free_segments(sbi));
-
 		/* if return value is not zero, no victim was selected */
 		if (f2fs_gc(sbi, test_opt(sbi, FORCE_FG_GC)))
 			wait_ms = gc_th->no_gc_sleep_time;
 
+		trace_f2fs_background_gc(sbi->sb, wait_ms,
+				prefree_segments(sbi), free_segments(sbi));
+
 		/* balancing f2fs's metadata periodically */
 		f2fs_balance_fs_bg(sbi);