filemap: Remove obsolete comment in lock_page

We no longer need the page's inode pinned.  This comment dates back to
commit db37648c ("[PATCH] mm: non syncing lock_page()") which added
lock_page_nosync().  That was removed by commit 7eaceacc ("block:
remove per-queue plugging") which also made this comment obsolete.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>