FBSD: avoid unused-variable warning

This variable is passed as an argument to the ma_vn_lock() compat
macro, which ignores the thread argument on some versions of FreeBSD.
Make the variable only be declared in those cases when it will be used.

Change-Id: I1ed10654fb402f4feec55d6d7c7ece6f0c78bc8e
Reviewed-on: http://gerrit.openafs.org/11373
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>