aio_md_test: label the md

Add a label to md devices created by this test.  The next time this
test leaks md devices, finding the culprit will be much easier.

Thanks to:	sobomax, for adding labels in r322969
MFC after:	1 week
Sponsored by:	Dell EMC Isilon

tests/sys/aio/aio_test.c

@@ -734,6 +734,8 @@ aio_md_setup(void)
 	mdio.md_options = MD_AUTOUNIT | MD_COMPRESS;
 	mdio.md_mediasize = GLOBAL_MAX;
 	mdio.md_sectorsize = 512;
+	strlcpy(buf, __func__, sizeof(buf));
+	mdio.md_label = buf;
 
 	if (ioctl(mdctl_fd, MDIOCATTACH, &mdio) < 0) {
 		error = errno;