Linux: Use atomics for credential reference counts

Linux: Use atomics for credential reference counts

The reference count maintained as part of the afs_cred structure
wasn't being maintained atomically, requiring that crfree and
crhold always be called with the GLOCK held.

This patch just switches to using Linux's inbuilt atomic types to
maintain the reference count.

Change-Id: I1787061afbb48e234b4839b38b8801168ea2f25f
Reviewed-on: http://gerrit.openafs.org/726
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>