vn_lock(9) on FreeBSD

The text b341d137f19e, as FreeBSD 5.5-RELEASE, FreeBSD 5.4-RELEASE, FreeBSD 5.3-RELEASE, FreeBSD 5.2.1-RELEASE, FreeBSD 5.2-RELEASE shipped it.

serialize access to a vnode

vn_lock(9) on FreeBSD now