userns: Fix posix_acl_file_xattr_userns gid conversion

The code needs to be from_kgid(make_kgid(...)...) not
from_kuid(make_kgid(...)...). Doh!

Reported-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>