volser: Fix bad readlink usage

readlink fills the buffer passed to it with a non-terminated string.
It can legitimately fill the whole of this buffer. So, if we require
a string to be NUL terminated, we must give readlink one less than
the string length so that the termination character can be safely
appended.

Caught by coverity (#985611)

Change-Id: Ie6c651a9c0fd94d83bd05395204341d71e353146
Reviewed-on: http://gerrit.openafs.org/9290
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>