net: ptp: fix infinite loop in pkt frag

There was wrong implementation in pkt frag removing and inserting.
This was causing infinite loop in either net_buf_frag_last or
net_pkt_get_len. This happened only when the pkt frag removing
and inserting was executed too fast after sending pkt before
ethernet_send calling net_pkt_get_len.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>

subsys/net/lib/ptp/msg.c

@@ -275,6 +275,7 @@ struct ptp_msg *ptp_msg_from_pkt(struct net_pkt *pkt)
 		/* remove packet temporarily. */
 		buf = pkt->buffer;
 		pkt->buffer = buf->frags;
+		buf->frags = NULL;
 
 		hdr = net_udp_get_hdr(pkt, NULL);