riscv pmap: another vm_radix_init

pmap_pinit0 also needs to initialize a vm_radix, in case vm_radix_init
does anything but zeroing fields.

Reported by:	alc
Reviewed by:	alc
Differential Revision:	https://reviews.freebsd.org/D41055

sys/riscv/riscv/pmap.c

@@ -1327,6 +1327,7 @@ pmap_pinit0(pmap_t pmap)
 	pmap->pm_satp = pmap_satp_mode() |
 	    (vtophys(pmap->pm_top) >> PAGE_SHIFT);
 	CPU_ZERO(&pmap->pm_active);
+	vm_radix_init(&pmap->pm_root);
 	pmap_activate_boot(pmap);
 }