libafs: Correct GUACB afs_LoopServers flags

Use the named constant AFS_LS_ALL.

This change is 1.6-specific, and contains only part of
1f48bc7a30d5df88a1e5f539ee22df3952533a88. The rest of
1f48bc7a30d5df88a1e5f539ee22df3952533a88 was committed in
4c4075ade0d46087638bd8cc96aa9fac005e62f3, before the FlushAllVCBs call
was added.

Reviewed-on: http://gerrit.openafs.org/4624
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 1f48bc7a30d5df88a1e5f539ee22df3952533a88)

Change-Id: I9284abe1cb9752c5204b9b1c91438983d854a864
Reviewed-on: http://gerrit.openafs.org/8840
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>