afs: Refactor DoLockWarning

Change DoLockWarning around a little bit, so subsequent changes are
easier to follow. Move lastWarnTime/lastWarnPid so they are only
usable within this function.

This commit should incur no functional change.

Change-Id: I5d25f64e9c088aecee0f0c46b6c401b2caa71ccb
Reviewed-on: http://gerrit.openafs.org/10794
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>