up: Fix improper use of readlink

readlink returns a non-NUL terminated string. If the string must be
terminated, we need to have space in the buffer for it. So, the
buffer passed to readlink must be 1 less than the real length of
the buffer.

Caught by coverity (#985597, #985610)

Change-Id: Id9a6f8227d756140960ac295330aaf17dca2ec03
Reviewed-on: http://gerrit.openafs.org/9329
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>