Make this function more generic by passing the file instead of the iocb.
Check in the callers whether we should call readpage or not.  Also make it
return an errno / 0 / AOP_TRUNCATED_PAGE, and make calling put_page() the
caller's responsibility.

Link: https://lkml.kernel.org/r/20210122160140.223228-10-willy@infradead.orgSigned-off-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: NKent Overstreet <kent.overstreet@gmail.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>