viced: prevent writes on readonly fileservers

viced: prevent writes on readonly fileservers

Currently, a fileserver can be initialized as readonly. In this mode,
writes on this server should not be allowed. Unfortunately, updates on
files stored by readonly fileservers are not completely prevented. In
some situations, the check for RO server is omitted (e.g. if the user is
the owner of the file to be updated). In other situations, the same
check is redundant.

To fix these problems, consolidate this check in one place.

Reviewed-on: https://gerrit.openafs.org/13934
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 0593017177edd5b3bc6609d9dfcce55f15bba3e9)

Change-Id: I42034928d1f5e9342029121613ac8d716818c3ae
Reviewed-on: https://gerrit.openafs.org/14018
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>