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.

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>
(cherry picked from commit 6ca324e565c34d9d04f3c553b7d0febe675ae538)

Change-Id: I82803669dd34d7abeb29040fbb38ec2f000f2601
Reviewed-on: http://gerrit.openafs.org/11658
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>