f2fs: introduce f2fs_flush_merged_bios for cleanup

Add a new helper f2fs_flush_merged_bios to clean up redundant codes.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -583,11 +583,8 @@ static void f2fs_put_super(struct super_block *sb)
 	mutex_unlock(&sbi->umount_mutex);
 
 	/* our cp_error case, we can wait for any writeback page */
-	if (get_pages(sbi, F2FS_WRITEBACK)) {
-		f2fs_submit_merged_bio(sbi, DATA, WRITE);
-		f2fs_submit_merged_bio(sbi, NODE, WRITE);
-		f2fs_submit_merged_bio(sbi, META, WRITE);
-	}
+	if (get_pages(sbi, F2FS_WRITEBACK))
+		f2fs_flush_merged_bios(sbi);
 
 	iput(sbi->node_inode);
 	iput(sbi->meta_inode);