net: net_app: set packet data_len on incoming packets

This fixes a bug when calling net_pkt_append[_all] which uses
pkt->data_len as part of the maximum packet length calculation
when the net_context is set.

Without this change the maximum packet length is calculated as 0
(the value of pkt->data_len) and an ENOMEM error is returned.

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>

subsys/net/lib/app/net_app.c

@@ -2068,6 +2068,7 @@ reset:
 				ctx->tls.mbedtls.ssl_ctx.hdr = NULL;
 			}
 
+			pkt->data_len = len;
 			ret = net_pkt_append_all(pkt, len,
 						 ctx->tls.request_buf,
 						 BUF_ALLOC_TIMEOUT);