powerpc pmap: initialize kernel pmap radix trie

Commit 2d2bcba7ba70141388729ed49674b36fd01146c5 changed radix trie
implementation and made it necessary that radix tries be initialized
with vm_radix_init. @dbaio reports that in some configurations, there
is a powerpc boot panic and that this commit introduced the
problem. In powerpc/aim/mmu_radix.c, the radix trie in kernel_pmap is
initialized by zeroing all its fields.

Add a call to vm_radix_init to properly initialize
kernel_pmap->pm_radix.

Reported by: dbaio
Reviewed by: alc, jhibbits
Fixes: 2d2bcba7ba drop NULL check from radix trie search
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D41846

(cherry picked from commit 8763b9862f90dc3bdbbc5e3c2664890be7c1f0c4)