afsd: check if mountdir is an absolute path

if the mountdir in the cacheinfo file is not absolute,
it can confuse commands like "df". Thus, force it to
be absolute.

Change-Id: Idb098b7c83fef6931fe71dd53a85569a953e5e3f
Reviewed-on: http://gerrit.openafs.org/10250
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>