Linux: Use the kernel's credentials structure

Linux: Use the kernel's credentials structure

Recent kernels (2.6.29 and above) have a separate ref-counted
structure for holding credentials.  Use it directly instead of
keeping a separate afs specific structure that shadows the same
information.

Also adapt Linux for the change from cr_xxx to afs_cr_xxx wrappers.

Reference counting is done with the appropriate get/put calls.

Change-Id: I1135bb5a66cda51cee4fa9f3f3e63eaa9af28f61
Reviewed-on: http://gerrit.openafs.org/797
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>