libext2: advance group in ext2fs_open2 during swapping

Without this change, we go back to getting group descriptor "0" each time we go around the "for i" loop. It must properly advance through the filesystem.

---

(I couldn't decipher the removed comment; maybe it was typed when you thought you would do:

                        gdp = ext2fs_group_desc(fs, dest, j);

which would also work ... ? It'd be clever, but I think my change is more explicitly obvious. Either way works.

We still have failing tests but at least nothing hangs now:
Tests failed: f_mmp f_mmp_garbage m_mmp t_mmp_1on t_mmp_2off

index 0cefe3f..40a52c5 100644
@@ -356,8 +356,8 @@ errcode_t ext2fs_open2(const char *name, const char *io_options,
 			goto cleanup;
 #ifdef WORDS_BIGENDIAN
 		for (j=0; j < groups_per_block; j++) {
-			/* The below happens to work... be careful. */
-			gdp = ext2fs_group_desc(fs, fs->group_desc, j);
+			gdp = ext2fs_group_desc(fs, fs->group_desc,
+						i * groups_per_block + j);
 			ext2fs_swap_group_desc2(fs, gdp);
 		}
 #endif

Thanks, applied.

- Ted