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)

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>
(cherry picked from commit 80875f8aa1540bf032073a4843566bbe6b1fe301)

Change-Id: I1dd039bfceaff7891c9145cf4799469b2de2f23a
Reviewed-on: http://gerrit.openafs.org/9371
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>