mm/filemap: don't call ->readpage if IOCB_WAITQ is set

The readpage operation can block in many (most?) filesystems, so we should
punt to a work queue instead of calling it.  This was the last caller of
lock_page_for_iocb(), so remove it.

Link: https://lkml.kernel.org/r/20210122160140.223228-9-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Kent Overstreet <kent.overstreet@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>