set_global_var_parse_kv() should pass the pointer from strdup()

A comment says that the caller should free k_out, but the pointer passed
via k_out is not the same pointer we received from strdup(). Instead,
it is a pointer into the region we received from strdup(). The free
function should always be called with the original pointer, so this is
likely a bug.

We solve this by calling `strdup()` a second time and then freeing the
original pointer.

Coverity reported this as a memory leak.

Reviewed-by: Neal Gompa <ngompa@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes openzfs#13867

lib/libzpool/util.c

@@ -173,12 +173,13 @@ set_global_var_parse_kv(const char *arg, char **k_out, u_longlong_t *v_out)
 		goto err_free;
 	}
 
-	*k_out = k;
+	*k_out = strdup(k);
 	*v_out = val;
+	free(d);
 	return (0);
 
 err_free:
-	free(k);
+	free(d);
 
 	return (err);
 }