VNode sequencing and locking - part 3/4.

VNode sequencing and locking - part 3/4.

VNode aliasing is handled by the namecache (aka nullfs), so there is no
longer a need to have VOP_LOCK, VOP_UNLOCK, or VOP_ISSLOCKED as 'VOP'
functions.  Both NFS and DEADFS have been using standard locking functions
for some time and are no longer special cases.  Replace all uses with
native calls to vn_lock, vn_unlock, and vn_islocked.

We can't have these as VOP functions anyhow because of the introduction of
the new SYSLINK transport layer, since vnode locks are primarily used to
protect the local vnode structure itself.