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.

Change-Id: Ia6a341e96d00071587443f8eef4a99992b368abc
Reviewed-on: http://gerrit.openafs.org/8980
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>