mm/filemap: fix infinite loop in generic_file_buffered_read()

If iter->count is 0 and iocb->ki_pos is page aligned, this causes
nr_pages to be 0.

Then in generic_file_buffered_read_get_pages() find_get_pages_contig()
returns 0 - because we asked for 0 pages, so we call
generic_file_buffered_read_no_cached_page() which attempts to add a page
to the page cache, which fails with -EEXIST, and then we loop. Oops...
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Reported-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -2453,6 +2453,9 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 
 	if (unlikely(iocb->ki_pos >= inode->i_sb->s_maxbytes))
 		return 0;
+	if (unlikely(!iov_iter_count(iter)))
+		return 0;
+
 	iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
 
 	if (nr_pages > ARRAY_SIZE(pages_onstack))