afs: Suppress duplicate message on Linux

Update afs_util.c to call afs_warnall in places where afs_warn and
afs_warnuser are called back to back with identical parameters.

Reviewed-on: http://gerrit.openafs.org/11006
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit 0287086fa78f0bc0012325c2eadb3dccb6b1f766)

Change-Id: I91d13654fbedffac4b2ad7ef54a90ade245de726
Reviewed-on: http://gerrit.openafs.org/11183
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>