When ns->level is not larger then cred->user_ns->level,
then ns can't be cred->user_ns's descendant, and
there is no a sense to search in parents.

So, break the cycle earlier and skip needless iterations.

v2: Change comment on suggested by Andy Lutomirski.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>