cifs.upcall: don't do env scraping when uid is 0

cifs.upcall: don't do env scraping when uid is 0

Setuid programs triggering upcalls could trick the program here. Also,
the d_automount method is done with credentials overridden so if you
can end up with mismatched creds and env vars due to that as well.

It's a hack, but the only recourse I can see is to avoid doing this
when the uid is 0. That means we can't rely on finding root credcaches
in alternate locations using $KRB5CCNAME, but I think that's the best
we can do.

Reported-and-Tested-by: Chad William Seys <cwseys@physics.wisc.edu>
Signed-off-by: Jeff Layton <jlayton@samba.org>