Remove hardcoded maximum time

Remove hardcoded maximum time

When iterating across the buffer list, afs_newslot used a hardcoded
maximum time to find the oldest. Instead of using this, just use the
accesstime of the first unused buffer that we find as the oldest, and
continue as normal.

Change-Id: Ifeacb8a407901b46a23fbc838b1fca761108ebbb
Reviewed-on: http://gerrit.openafs.org/738
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>