Catch up to FreeBSD VM object read/write locks

Upstream r248084 changed the vm_object mutex to be a rwlock,
allowing for future optimizations.  This is a KPI change, so
introduce conditionals to be compatible with both versions of the KPI.

Reviewed-on: http://gerrit.openafs.org/10295
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 897e970dbe09d163479719b4c9befa660d99874b)

Change-Id: Ieeb30f3ad9accab37117109ccf82952812157aab
Reviewed-on: http://gerrit.openafs.org/10378
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>