SOLARIS: Use kcred instead of afs_osi_cred

SOLARIS: Use kcred instead of afs_osi_cred

For many vfs ops to the cache, we currently pass &afs_osi_cred for our
credentials, which is a mostly zeroed-out credential structure. In
some modern versions of Solaris (Solaris 11), at least some parts of
this structure need to not be NULL (cr_zone), or we will panic.

The Solaris kernel provides a 'kcred' credentials structure for the
purpose of using "kernel" credentials for i/o. So just use that
instead for Solaris 8 and beyond, since kcred has existed at least
since Solaris 8.

Reviewed-on: http://gerrit.openafs.org/6669
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit dc6beb3ea29a64bcf59807fd451a573aa54e1122)

Change-Id: I6fd0ce4a890c2e6d9377cad39f47303aa1687a6b
Reviewed-on: http://gerrit.openafs.org/6682
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>