Introduce a global vnode_mtx and use it to make vn_lock() safe to be called

Introduce a global vnode_mtx and use it to make vn_lock() safe to be called
without the KERNEL_LOCK.
This moves VXLOCK and VXWANT to a mutex protected v_lflag field and also
v_lockcount is protected by this mutex.

The vn_lock() dance is overly complex and all of this should probably replaced
by a proper lock on the vnode but such a diff is a lot more complex. This
is an intermediate step so that at least some calls can be modified to grab
the KERNEL_LOCK later or not at all.

OK mpi@