quota: Handle quota data stored in s_user_ns in quota_setxquota

In Q_XSETQLIMIT use sb->s_user_ns to detect when we are dealing with
the filesystems notion of id 0.

Cc: Jan Kara <jack@suse.cz>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Inspired-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>