Reduce lock contention on the z_teardown_lock under heavily cached
read workload by splitting the single teardown rrw lock into
RRM_NUM_LOCKS (17) of them.

Read acquisitions are randomly distributed among these locks based
on curthread pointer.  Write acquisitions are going to all the
locks, which for the usage of this type of lock should be rare.

Illumos issue:
    5008 lock contention (rrw_exit) while running a read only load

MFC after:	2 weeks