Linux Keyring error handling

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

Change-Id: Ia62c1c24b22e833cd5dc2689181397965901d34e
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>