squashfs: xattr simplifications

Now that the xattr handler is passed to the xattr handler operations, we
have access to the attribute name prefix, so simplify the squashfs xattr
handlers a bit.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Phillip Lougher <phillip@squashfs.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>