[PowerPC] Fix 32-bit Book-E panic due to pve leak

On an INVARIANTS kernel on 32-bit Book-E, we were panicing when running
the libproc tests. This was caused by extra pv entries being generated
accidentally by the pmap icache invalidation code.

Use the same VA (i.e. 0) when freeing the temporary mapping, instead of
some arbitrary address within the zero page.

Failure to do this was causing kernel-side icache syncing to leak
PVE entries when invalidating icache for a non page-aligned address, which
would later result in pages erroneously showing up as mapped to vm_page.

This bug was introduced in r347354 in 2019.

Reviewed by:	jhibbits (in irc)
Sponsored by:	Tag1 Consulting, Inc.

sys/powerpc/booke/pmap_32.c

@@ -748,14 +748,23 @@ mmu_booke_sync_icache(pmap_t pm, vm_offset_t va, vm_size_t sz)
 		sync_sz = min(sync_sz, sz);
 		if (valid) {
 			if (!active) {
-				/* Create a mapping in the active pmap. */
+				/*
+				 * Create a mapping in the active pmap.
+				 *
+				 * XXX: We use the zero page here, because
+				 * it isn't likely to be in use.
+				 * If we ever decide to support
+				 * security.bsd.map_at_zero on Book-E, change
+				 * this to some other address that isn't
+				 * normally mappable.
+				 */
 				addr = 0;
 				m = PHYS_TO_VM_PAGE(pa);
 				PMAP_LOCK(pmap);
 				pte_enter(pmap, m, addr,
 				    PTE_SR | PTE_VALID, FALSE);
-				addr += (va & PAGE_MASK);
-				__syncicache((void *)addr, sync_sz);
+				__syncicache((void *)(addr + (va & PAGE_MASK)),
+				    sync_sz);
 				pte_remove(pmap, addr, PTBL_UNHOLD);
 				PMAP_UNLOCK(pmap);
 			} else