f2fs: revisit the f2fs_gc flow

I'd like to revisit the f2fs_gc flow and rewrite as follows.

1. In practical, the nGC parameter of f2fs_gc is meaningless. So, let's
  remove it.
2. Background GC marks victim blocks as dirty one at a time.
3. Foreground GC should do cleaning job until acquiring enough free
  sections. Afterwards, it needs to do checkpoint.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/segment.c

@@ -31,7 +31,7 @@ void f2fs_balance_fs(struct f2fs_sb_info *sbi)
 	 */
 	if (has_not_enough_free_secs(sbi)) {
 		mutex_lock(&sbi->gc_mutex);
-		f2fs_gc(sbi, 1);
+		f2fs_gc(sbi);
 	}
 }