nvme-print-stdout: change stdout_d() to not print all zeros

[ikegami-t]

This is for the id-ns command empty vendor-specific fields.
Note: This will change for other commands d() outputs.

[igaw]

why not going the full hexdump way?

static bool line_equal(const unsigned char *a,
		       const unsigned char *b,
		       size_t len)
{
	return memcmp(a, b, len) == 0;
}

void stdout_d(unsigned char *buf, int len, int width, int group)
{
	size_t offset, i;
	unsigned char prev[32];
	char ascii[32 + 1];
	bool have_prev = false;
	bool omitting = false;

	assert(width <= sizeof(prev));
	assert(width < sizeof(ascii));
	assert(group > 0);

	/* Header */
	printf("     ");
	for (i = 0; i < width; i++)
		printf("%3zx", i);
	printf("\n");

	for (offset = 0; offset < len; offset += width) {
		size_t line_len = width;

		if (offset + line_len > len)
			line_len = len - offset;

		/* Collapse identical lines (hexdump behavior) */
		if (have_prev &&
		    line_len == width &&
		    line_equal(buf + offset, prev, width)) {
			if (!omitting) {
				printf("*\n");
				omitting = true;
			}
			continue;
		}

		omitting = false;

		/* Save current line for next comparison */
		if (line_len == width) {
			memcpy(prev, buf + offset, width);
			have_prev = true;
		} else {
			have_prev = false;
		}

		printf("%04zx:", offset);
		memset(ascii, 0, sizeof(ascii));

		for (i = 0; i < line_len; i++) {
			if (i % group)
				printf("%02x", buf[offset + i]);
			else
				printf(" %02x", buf[offset + i]);

			ascii[i] = (buf[offset + i] >= '!' &&
				    buf[offset + i] <= '~') ?
				   buf[offset + i] : '.';
		}

		/* Pad short final line */
		if (line_len < width) {
			size_t missing = width - line_len;
			size_t pad = missing * 2 +
				     missing / group +
				     (missing % group ? 1 : 0);
			printf(" %*s", (int)pad, "");
		}

		printf(" \"%.*s\"\n", (int)line_len, ascii);
	}

	/* Final offset line (hexdump prints this always) */
	printf("%04zx\n", offset);
}

this gives

vs[]:
       0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
0000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 "................"
*
00c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 07 "................"
00d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 "................"
*
03a0: 01 00 3c 0f 00 00 00 00 00 00 00 00 00 00 00 00 "..<............."
03b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 "................"
*
0400

[ikegami-t]

Just realized as the hexdump not print equal line but not only for the all zero then fixed the patch as mentioned.

(Added)
Changed to dump all equal lines if -vv debug log level enabled by the additional patch.