Fix PAGs for all platforms without Linux keyrings

Fix PAGs for all platforms without Linux keyrings

The changes in 7b272177de4c6f78db7f2315f4e30e85ab7660bb sadly break
all platforms which aren't Linux. This is because the logic in PagInCred
doesn't handle the non-Linux case at all. Fix this so that we call
afs_get_group_pag whenever we're not running on Linux.

The code also doesn't match the comment - Linux kernels which don't
have keyrings should use afs_get_group_pag(), regardless of whether
STRUCT_TASK_HAS_CRED. It's unlikely this bit would ever bite anyone
though, kernels with cred support almost certainly require keyrings.

Change-Id: I5373da51151229dc13b6dc60c3cf10700f0f881a
Reviewed-on: http://gerrit.openafs.org/1006
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>