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.

Caught by coverity (#985596)

Change-Id: I47081877a54a7b3d99ab8e6ec52d4663acd2eeb6
Reviewed-on: http://gerrit.openafs.org/9328
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>