afs: Avoid tracking file locks for RO volumes

Advisory file locks for RO volumes don't make a lot of sense, since
there are no possible writes to worry about. The fileserver already
does not track these, so don't even bother processing them in the
client.

Reviewed-on: http://gerrit.openafs.org/8197
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 0fc27471e7da0c5de4addcdec1bfbca5208072cc)

Change-Id: I308d930c94708fd8d9025886c10f40cc4d10d195
Reviewed-on: http://gerrit.openafs.org/8910
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>