Linux: Move code to reset the root to afs/LINUX

Linux: Move code to reset the root to afs/LINUX

Move the Linux specific bit of code to reset the root to
afs/LINUX platform specific files.  Things that play with
the Linux vfs internals should not be exposed here.

No functional change, but this helps cleanup some ifdef
mess.

Change-Id: Ia27fca3d8052ead45783cb2332c04fe6e99e5d9f
Reviewed-on: http://gerrit.openafs.org/11641
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Laß <lass@mail.uni-paderborn.de>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>