afs: Fix afs_fs_fetch_data() to subtract transferred from len

afs: Fix afs_fs_fetch_data() to subtract transferred from len

Fix afs_fs_fetch_data() to subtract subreq->transferred from subreq->len
rather than adding it.

Fixes: f28fc2010d62 ("afs: Eliminate afs_read")
Link: https://sashiko.dev/#/patchset/20260713081022.2186481-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/20260723113452.566619-3-dhowells@redhat.com
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeffrey Altman <jaltman@auristor.com>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>