mm/filemap: fix infinite loop in generic_file_buffered_read()

mainline inclusion
from mainline-v5.11-rc1
commit 3644e2d2
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I40B5X


CVE: NA

-------------------------------------------------

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>

Signed-off-by: Nanyong Sun <sunnanyong@huawei.com>
Reviewed-by: Tong Tiangen <tongtiangen@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>