f2fs: avoid BG_GC in f2fs_balance_fs

If many threads hit has_not_enough_free_secs() in f2fs_balance_fs() at the same
time, all the threads would do FG_GC or BG_GC.
In this critical path, we totally don't need to do BG_GC at all.
Let's avoid that.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 39616de..9ea89ca 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -1849,7 +1849,7 @@
 		mutex_lock(&sbi->gc_mutex);
 	}
 
-	ret = f2fs_gc(sbi, sync);
+	ret = f2fs_gc(sbi, sync, true);
 out:
 	mnt_drop_write_file(filp);
 	return ret;