afsd: Trim trailing slashes on Linux mntent

afsd: Trim trailing slashes on Linux mntent

When we write a mount entry on Linux when mounting /afs, trim trailing
slashes on the mount path. Otherwise, the umount utility can get
slightly confused, and leave the /afs mount entry in /etc/mtab after
it's been unmounted.

For full correctness we should probably completely canonicalize the
path like the mount utility does, but it's unlikely that anyone will
provide significantly weird paths for cacheMountDir, so don't bother.

Change-Id: Ie8330f08918d52eee319dff5f6ad275c30164c67
Reviewed-on: http://gerrit.openafs.org/4442
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>