net: dsa: b53: fix error propagation from b53_fdb_dump()

The blamed commit replaced "return ret" statements in b53_fdb_dump()
with "break;" which jumps to the mutex_unlock() -> return 0 section.

This is notably problematic because it swallows errors from the
b53_fdb_copy() -> cb() path, and this will result in FDB dump truncation
when the netlink skb overflows - see commit 21b52fed92 ("net: dsa:
sja1105: fix broken backpressure in .port_fdb_dump").

Let's go back to "return ret". We don't need to preinitialize "ret" with
0, because the "do {} while" block guarantees we cannot reach the end of
the function without at least once calling b53_arl_search_wait(), which
will have initialized ret to some valid value.

Fixes: f7eb4a1c08 ("net: dsa: b53: serialize access to the ARL table")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20260812201121.2012356-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/b53/b53_common.c

@@ -2219,7 +2219,7 @@ int b53_fdb_dump(struct dsa_switch *ds, int port,
 
 	mutex_unlock(&priv->arl_mutex);
 
-	return 0;
+	return ret;
 }
 EXPORT_SYMBOL(b53_fdb_dump);