afs: Fix a use after free in afs_xattr_get_acl()

The "op" pointer is freed earlier when we call afs_put_operation().

Fixes: e49c7b2f ("afs: Build an abstraction around an "operation" concept")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Colin Ian King <colin.king@canonical.com>