mdmfs(8): use -o reserve with malloc-backed md(4)

Mentioned in mdconfig(8), malloc-backed md(4) can be unstable unless
required memory is allocated up front with -o reserve. Furthermore, panics
have been observed with md used in fstab on 12.0-RELEASE. Choose the stable
route and pass -o reserve.

Submitted by:	Paul Vixie
MFC after:	1 week

sbin/mdmfs/mdmfs.c

@@ -196,6 +196,7 @@ main(int argc, char **argv)
 				usage();
 			mdtype = MD_MALLOC;
 			have_mdtype = true;
+			argappend(&mdconfig_arg, "-o reserve");
 			break;
 		case 'm':
 			argappend(&newfs_arg, "-m %s", optarg);