Add missing data barriers after storeing a new valid pagetable entry.

When moving from an invalid to a valid entry we don't need to invalidate
the tlb, however we do need to ensure the store is ordered before later
memory accesses. This is because this later access may be to a virtual
address within the newly mapped region.

Add the needed barriers to places where we don't later invalidate the
tlb. When we do invalidate the tlb there will be a barrier to correctly
order this.

This fixes a panic on boot on ThunderX2 when INVARIANTS is turned off:
panic: vm_fault_hold: fault on nofault entry, addr: 0xffff000040c11000

Reported by:	jchandra
Tested by:	jchandra
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D19097

sys/arm64/arm64/pmap.c

@@ -2862,6 +2862,7 @@ pmap_update_entry(pmap_t pmap, pd_entry_t *pte, pd_entry_t newpte,
 
 	/* Create the new mapping */
 	pmap_load_store(pte, newpte);
+	dsb(ishst);
 
 	critical_exit();
 	intr_restore(intr);
@@ -3281,6 +3282,7 @@ pmap_enter(pmap_t pmap, vm_offset_t va, vm_page_t m, vm_prot_t prot,
 	} else {
 		/* New mappig */
 		pmap_load_store(l3, new_l3);
+		dsb(ishst);
 	}
 
 #if VM_NRESERVLEVEL > 0
@@ -3435,6 +3437,7 @@ pmap_enter_l2(pmap_t pmap, vm_offset_t va, pd_entry_t new_l2, u_int flags,
 	 * Map the superpage.
 	 */
 	(void)pmap_load_store(l2, new_l2);
+	dsb(ishst);
 
 	atomic_add_long(&pmap_l2_mappings, 1);
 	CTR2(KTR_PMAP, "pmap_enter_l2: success for va %#lx in pmap %p",