MFC r313454,r313472:

rwlock: implemenet rlock/runlock fast path

This improves singlethreaded throughput on my test machine from ~247 mln
ops/s to ~328 mln.

It is mostly about avoiding the setup cost of lockstat.

rwlock: fix r313454

The runlock slow path would update wrong variable before restarting the
loop, in effect corrupting the state.