fuse: Set *nbytesp=0 in fuse_get_user_pages on allocation failure

In fuse_get_user_pages(), set *nbytesp to 0 when struct page **pages
allocation fails. This prevents the caller (fuse_direct_io) from making
incorrect assumptions that could lead to NULL pointer dereferences
when processing the request reply.

Previously, *nbytesp was left unmodified on allocation failure, which
could cause issues if the caller assumed pages had been added to
ap->descs[] when they hadn't.

Reported-by:  <syzbot+87b8e6ed25dbc41759f7@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=87b8e6ed25dbc41759f7


Fixes: 3b97c365 ("fuse: convert direct io to use folios")
Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Tested-by: Dmitry Antipov <dmantipov@yandex.ru>
Tested-by: David Howells <dhowells@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>