Revert previous attempt at serializing `uo_refs'.

Revert previous attempt at serializing `uo_refs'.

It is incorrect to look at/modify `uo_refs' without lock in uvm_vnp_sync().
However sleeping is not possible and spinning introduces a deadlock with the
KERNEL_LOCK() when another thread faults on a mmaped region related to a
vnode we're currently synching.

A proper fix should be investigated.  For now a kown a race, mostly safe due
to the KERNEL_LOCK(), is better than a possible deadlock.

Issue reported by caspar@

Discussed with and ok semarie@