Linux: setpag() may replace credentials

For recent Linux. setpag() may replace the current process' cred
structure with a new one.  This is not a problem for most callers,
but in the case of processing a SetTokens2 pioctl with the setpag
option, the new credentials should be used to determine the target
for the token.

Reviewed-on: http://gerrit.openafs.org/8924
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit b61eac783e8c092cd4ba9f53a2b5ca7d43e08b1b)

Change-Id: I66b5f171318964ff40fe78be24e75519183c3a82
Reviewed-on: http://gerrit.openafs.org/8948
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>