Linux: crset should use passed credentials

When copying credentials in crset, take the passed credentials as
the source instead of the current process credentials.

This doesn't cause any problems currently since crset always gets
called with the current credentials.

Reviewed-on: http://gerrit.openafs.org/8980
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 3b733db8bf2bc3a1366554ae97814cc722437149)

Change-Id: I4d0000a139d16fafe480f7ebccc6759da7dea6bb
Reviewed-on: http://gerrit.openafs.org/9333
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>