Catch up with FBSD80's removal of thread argument to VFS calls

Catch up with FBSD80's removal of thread argument to VFS calls

FreeBSD SVN rev 191990 removed the thread argument to
filesystem-dependent VFS calls.  Curthread is always assumed.
There is a __FreeBSD_version bump that we could check if
we felt like being precise, but FBSD80_ENV is probably good enough.

Change-Id: I4534f01999b61eaeaa014c41473c693e6d432fef
Reviewed-on: http://gerrit.openafs.org/1587
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>