util/nvmutil: cmd_swap(): write sequentually

the current code writes part 1 first, and part 0 next,
on the disk, due to the way the swap works.

with this change, swap still swaps the two parts of the file,
on disk, but writes the new file sequentially.

this change might speed up i/o on the file system, on HDDs.
on SSDs, this change likely makes no difference at all.

util/nvmutil/nvmutil.c

@@ -438,16 +438,21 @@ writeGbeFile(int *fd, const char *filename)
 		nw = 128;
 
 	for (p = 0; p < 2; p++) {
+		if (gbe[0] > gbe[1])
+			p ^= 1;
 		if (nvmPartModified[p]) {
 			printf("Part %d modified\n", p);
 		} else {
 			fprintf (stderr,
 				"Part %d NOT modified\n", p);
-			continue;
+			goto next_part;
 		}
 		if (pwrite((*fd), (uint8_t *) gbe[p], nw, p << 12) != nw)
 			err(errno, "%s", filename);
 		tw += nw;
+next_part:
+		if (gbe[0] > gbe[1])
+			p ^= 1;
 	}
 	if (close((*fd)))
 		err(errno, "%s", filename);