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
r243637 changed the default number of nfsd threads created,
but the man page did not reflect this.
This is a content change.

Reported by:	bde
MFC after:	2 weeks
Use unsigned optlen in getsourcefilter()

Sizes can not be negative and the functions that use it
expect an unsigned value anyways.

Obtained from:	Apple (Libc-997.90.3)
MFC after:	1 week