Linux Keyring error handling

honour the returncode of key_instantiate_and_link() to avoid
having non-working pagsh without an error.

Reviewed-on: http://gerrit.openafs.org/10179
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit c7f5a212025172307e85e09ada03333e5e288cc3)

Change-Id: Ib5809e36ae8e87190206d75286bbeb23ace34b25
Reviewed-on: http://gerrit.openafs.org/10255
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>