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.

Change-Id: I4441349b8a72eac7d56276abcdf243b2ffd21b98
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>