f2fs: Fix __write_node_folio() conversion

This conversion moved the folio_unlock() to inside __write_node_folio(),
but missed one caller so we had a double-unlock on this path.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Chao Yu <chao@kernel.org>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Reported-by:  <syzbot+c0dc46208750f063d0e0@syzkaller.appspotmail.com>
Fixes: 80f31d2a (f2fs: return bool from __write_node_folio)
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>