libnvme/json: check write return value for build warnings

[ikegami-t]

Since the following build warnings output.

[61/232] Compiling C object libnvme/src/libnvme-test.so.p/nvme_json.c.o ../libnvme/src/nvme/json.c: In function ‘json_update_config’: ../libnvme/src/nvme/json.c:438:9: warning: ignoring return value of ‘write’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
438 | write(fd, "\n", 1);
| ^~~~~~~~~~~~~~~~~~
[62/232] Compiling C object libnvme/src/libnvme.so.3.0.0.p/nvme_json.c.o
../libnvme/src/nvme/json.c: In function ‘json_update_config’:
../libnvme/src/nvme/json.c:438:9: warning: ignoring return value of ‘write’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
438 | write(fd, "\n", 1);
| ^~~~~~~~~~~~~~~~~~

Fixes: 74b30d2 ("json: update nvme_dump_config to a file descriptor")

[igaw]

The log message could then be missleading when json_object_to_fd is succesfull and then the write fails. In this case json_util_get_last_error will not be correct.

something like this here would be less confusing

	ret = json_object_to_fd(fd, json_root,
				JSON_C_TO_STRING_PRETTY |
				JSON_C_TO_STRING_NOSLASHESCAPE);
	if (ret < 0) {
		nvme_msg(ctx, LOG_ERR, "Failed to write JSON config file: %s\n",
			 json_util_get_last_err());
		[...]
	};

	ret = write(...)
	if (ret < 0) {
		nvme_msg(...);
	}

(though highly unlikely to happen)

[ikegami-t]

How about this?

		nvme_msg(ctx, LOG_ERR, "Failed to write JSON config file: %s\n",
			  ret ? json_util_get_last_err() : strerror());

[igaw]

yes, that's fine too.

[ikegami-t]

Just fixed as commented. Thank you.

[igaw]

Thanks!