gpart: "gpart --libxo:JP list" duplicates attribute keys

Add leaf-list modifier to attrib as it's possible to have multiple
attributes on a gpart provider.

I purposely made it so that the normal output still says "attrib:" just
so we don't break any scripts people may be using to parse the output,
but the libxo output now says "attribute" just like `gpart show` will do
once https://reviews.freebsd.org/D53950 is merged.

PR:		291377
MFC after:	1 week
Reviewed by:	asomers
Sponsored by:	ConnectWise
Differential Revision: https://reviews.freebsd.org/D54080

sbin/geom/core/geom.c

@@ -912,8 +912,12 @@ list_one_provider(struct gprovider *pp, const char *padding)
 	}
 	xo_emit("{P:/%s}{Lcw:Mode}{:mode}\n", padding, pp->lg_mode);
 	LIST_FOREACH(conf, &pp->lg_config, lg_config) {
-		xo_emit("{P:/%s}{Lcwa:}{a:}\n", padding, conf->lg_name,
-		    conf->lg_name, conf->lg_val ? conf->lg_val : "");
+		if (strcmp(conf->lg_name, "attrib") != 0)
+			xo_emit("{P:/%s}{Lcwa:}{a:}\n", padding, conf->lg_name,
+			    conf->lg_name, conf->lg_val ? conf->lg_val : "");
+		else
+			xo_emit("{P:/%s}{Lcw:attrib}{l:attribute}\n", padding,
+			    conf->lg_val ? conf->lg_val : "");
 	}
 }