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>
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 62b52f2..8e5ca28 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -36,6 +36,14 @@
 	}
 }
 
+void f2fs_balance_fs_bg(struct f2fs_sb_info *sbi)
+{
+	/* check the # of cached NAT entries and prefree segments */
+	if (try_to_free_nats(sbi, NAT_ENTRY_PER_BLOCK) ||
+				excess_prefree_segs(sbi))
+		f2fs_sync_fs(sbi->sb, true);
+}
+
 static void __locate_dirty_segment(struct f2fs_sb_info *sbi, unsigned int segno,
 		enum dirty_type dirty_type)
 {