ck_swlock_read_latchlocks observers a writer if the unlatch
operation sets n_readers to 0.
The unlatch operation now just unsets the latch bit, we can safely
decrement n_readers in ck_swlock_read_latchlocks().
+ Fixes to validation tests & ELIDE coverage.
This lock is copy-safe when the latch operations are used.
Simplified write side operations lead to lower latencies than ck_rwlock
for single writer workloads.
Jaidev Sridhar
Samy Al Bahra