afs: Fix the afs.cell and afs.volume xattr handlers

Fix the ->get handlers for the afs.cell and afs.volume xattrs to pass the
source data size to memcpy() rather than target buffer size.

Overcopying the source data occasionally causes the kernel to oops.

Fixes: d3e3b7ea ("afs: Add metadata xattrs")
Signed-off-by: David Howells <dhowells@redhat.com>