By moving the iocb flag checks to the caller, we can pass the file and the
page index instead of the iocb.  It never needed the iter.  By passing the
pagevec, we can return an errno (or AOP_TRUNCATED_PAGE) instead of an
ERR_PTR.

Link: https://lkml.kernel.org/r/20210122160140.223228-11-willy@infradead.orgSigned-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>