Linux: Use current_creds() if stashed creds fail

Linux: Use current_creds() if stashed creds fail

If we can't use our stashed credentials (because SELinux hates us,
most likely), then fall back to trying the processes current credentials
instead of just oopsing.

Change-Id: Ib3676ff66f8400e86cccb97fb207a5f094ceb4ad
Reviewed-on: http://gerrit.openafs.org/1283
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit bcdb5220e2db2bf0cd22c6316682046425048d87)
Reviewed-on: http://gerrit.openafs.org/1289