Remove unneeded splvm() calls and the pool_setipl(9) hack of r1.140.

Remove unneeded splvm() calls and the pool_setipl(9) hack of r1.140.

By instructing spl(9) calls on MP machines I figured out that their high
cost was hiding a race condition involving PTE reuse in our pmap.  Thanks
to deraadt@ for finding a way to trigger such panic by adding a couple of
splvm().

This should make the races easier to trigger but will be addressed
shortly.

This commit starts your PowerPC pmap SMP show of the week.

ok kettenis@, deraadt@, dlg@