mpers.awk: relax union member name absence check

This fixes mpersing of unions containing nameless members,
e.g. struct btrfs_ioctl_vol_args_v2.

* mpers.awk (what_is): Print names of union_type members as is.

mpers.awk

@@ -146,7 +146,7 @@ function what_is(what_idx, type_idx, special, item, \
 			if ("parent" in array[item] && \
 				array_get(item, "parent") == what_idx) {
 				returned = what_is(item)
-				printf("%s", array_get(item, "name"))
+				printf("%s", array[item]["name"])
 				if ("" != returned) {
 					printf("[%s]", returned)
 				}