iov_iter: fix advancing slot in iter_folioq_get_pages()

iter_folioq_get_pages() decides to advance to the next folioq slot when
it has reached the end of the current folio. However, it is checking
offset, which is the beginning of the current part, instead of
iov_offset, which is adjusted to the end of the current part, so it
doesn't advance the slot when it's supposed to. As a result, on the next
iteration, we'll use the same folio with an out-of-bounds offset and
return an unrelated page.

This manifested as various crashes and other failures in 9pfs in drgn's
VM testing setup and BPF CI.

Fixes: db0aa2e956 ("mm: Define struct folio_queue and ITER_FOLIOQ to handle a sequence of folios")
Link: https://lore.kernel.org/linux-fsdevel/20240923183432.1876750-1-chantr4@gmail.com/
Tested-by: Manu Bretelle <chantr4@gmail.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Link: https://lore.kernel.org/r/cbaf141ba6c0e2e209717d02746584072844841a.1727722269.git.osandov@fb.com
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Tested-by: Leon Romanovsky <leon@kernel.org>
Tested-by: Joey Gouly <joey.gouly@arm.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

lib/iov_iter.c

@@ -1033,7 +1033,7 @@ static ssize_t iter_folioq_get_pages(struct iov_iter *iter,
 		if (maxpages == 0 || extracted >= maxsize)
 			break;
 
-		if (offset >= fsize) {
+		if (iov_offset >= fsize) {
 			iov_offset = 0;
 			slot++;
 			if (slot == folioq_nr_slots(folioq) && folioq->next) {