if_wg.c: Zero Padding Missing MTU Check

[Zixu Wu]

Zero padding shouldn't cause the packet size to go beyond MTU.

diff --git a/sys/net/if_wg.c b/sys/net/if_wg.c
index 0641c4b5ba9..90ef5b808a0 100644
--- a/sys/net/if_wg.c
+++ b/sys/net/if_wg.c
@@ -1592,6 +1592,9 @@ wg_encap(struct wg_softc *sc, struct mbuf *m)
        peer = t->t_peer;

        plaintext_len = WG_PKT_WITH_PADDING(m->m_pkthdr.len);
+       if (m->m_pkthdr.len <= t->t_mtu && plaintext_len > t->t_mtu) {
+               plaintext_len = t->t_mtu;
+       }
        padding_len = plaintext_len - m->m_pkthdr.len;
        out_len = sizeof(struct wg_pkt_data) + plaintext_len +
            NOISE_AUTHTAG_LEN;