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, since kcred has existed at least since Solaris 8.

Change-Id: Ia5252580d2de6dd7adfa1a1929148362d1da6360
Reviewed-on: http://gerrit.openafs.org/6669
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>