fs: Fix improper use of readlink

readlink returns a non-NUL terminated buffer. If we are going to
terminate its response, we need to make sure that there's space to
do so. So the length passed to readlink should be one less than the
real length of the buffer.

This is a 1.6-only change

It is inspired by commit 25011b4544e48dffd5978201669f7b9e23da3144
which cannot be cherry picked due to other missing commits on
the 1.6 branch, like 763ec5250deef6f0f8ae1c85b52bac3ffc1a6176.

Change-Id: I5c3f2946f7c1b8e935ae46bc195f58e207e115d6
Reviewed-on: http://gerrit.openafs.org/11054
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>