Linux - Fix disk cache access for selinux/AppArmor constrained processes

Linux - Fix disk cache access for selinux/AppArmor constrained processes

Preserve the credentials used for cache initialisation and use then
whenever disk cache files are opened.  This takes advantage of the
credentials separation work from David Howells available in kernels
2.6.29 and above.
Access to cache files was done under the security context of the
user process, causing processes constrained by selinux or AppArmor to
fail to access AFS cache files and causing the cache manager to panic.

Besides the RT tickets, should also fix the following Ubuntu bugs:
 415766 429260 457779 459299

FIXES 92944,125544

Change-Id: Ief8acd65c1a3e4d8c951f80bfd65f8340b8cec34
Reviewed-on: http://gerrit.openafs.org/752
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>
Reviewed-on: http://gerrit.openafs.org/774