Fix a low-memory deadlock in the VM system which can occur on systems

Fix a low-memory deadlock in the VM system which can occur on systems
with small amounts of system memory.  The pageout daemon uses LK_TIMELOCK
to avoid deadlocking on a vnode so it can continue operation.  This only
works if lk_timo is non-zero, but all filesystems initialize their vnodes
with a timeout of zero so the feature is defeated.  Adjust allocvnode()
to guarantee that lk_timo is never zero.