Add vmem locking to r281026.

While races there are not fatal, they cause result underestimation, that
cause unneeded ARC reclaims.

MFC after:	1 month

sys/kern/subr_vmem.c

@@ -1330,12 +1330,15 @@ vmem_size(vmem_t *vm, int typemask)
 	case VMEM_FREE|VMEM_ALLOC:
 		return vm->vm_size;
 	case VMEM_MAXFREE:
+		VMEM_LOCK(vm);
 		for (i = VMEM_MAXORDER - 1; i >= 0; i--) {
 			if (LIST_EMPTY(&vm->vm_freelist[i]))
 				continue;
+			VMEM_UNLOCK(vm);
 			return ((vmem_size_t)ORDER2SIZE(i) <<
 			    vm->vm_quantum_shift);
 		}
+		VMEM_UNLOCK(vm);
 		return (0);
 	default:
 		panic("vmem_size");