FBSD: lock interlock around v_usecount accesses

FBSD: lock interlock around v_usecount accesses

The FreeBSD vnode locking strategy requires that the vnode
interlock be held for all accesses to v_usecount, such as those
used by our VREFCOUNT and VREFCOUNT_GT macros.  Conveniently,
a wrapper function is provided that takes the lock around its
access of the element, vrefcnt().  Use it for our macros.

Change-Id: Ia88eb21046fe844a6adb830734dcae2f76ef2dc5
Reviewed-on: http://gerrit.openafs.org/3195
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>