FreeBSD: Catch up with recent VFS changes
cn_thread is always curthread.
https://cgit.freebsd.org/src/commit?id=b4a58fbf640409a1e507d9f7b411c83a3f83a2f3
https://cgit.freebsd.org/src/commit?id=2b68eb8e1dbbdaf6a0df1c83b26f5403ca52d4c3
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Alan Somers <asomers@gmail.com>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #12668