Revert "Reduce dbuf_find() lock contention"

This reverts commit 34dbc618f50cfcd392f90af80c140398c38cbcd1. While this
change resolved the lock contention observed for certain workloads, it
inadventantly reduced the maximum hash inserts/removes per second. This
appears to be due to the slightly higher acquisition cost of a rwlock vs
a mutex.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>