f2fs: introduce f2fs_balance_fs_bg for some background jobs

This patch merges some background jobs into this new function.

Signed-off-by: Changman Lee <cm224.lee@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/gc.c

@@ -83,9 +83,8 @@ static int gc_thread_func(void *data)
 		if (f2fs_gc(sbi))
 			wait_ms = gc_th->no_gc_sleep_time;
 
-		/* balancing prefree segments */
-		if (excess_prefree_segs(sbi))
-			f2fs_sync_fs(sbi->sb, true);
+		/* balancing f2fs's metadata periodically */
+		f2fs_balance_fs_bg(sbi);
 
 	} while (!kthread_should_stop());
 	return 0;