gnrc_ipv6: make next-header demuxer private

Since the recursion into `gnrc_ipv6_demux()` was removed in
`gnrc_ipv6_ext`, `gnrc_ipv6.c` is the only user of this function,
so it can be made private. It was only made public so it can be used
from `gnrc_ipv6_ext`.

sys/include/net/gnrc/ipv6.h

@@ -127,32 +127,6 @@ extern fib_table_t gnrc_ipv6_fib_table;
  */
 kernel_pid_t gnrc_ipv6_init(void);
 
-/**
- * @brief   Demultiplexes a packet according to @p nh.
- *
- * @internal
- *
- * **Do not use outside this module or its submodules!!!**
- * Public access needed for Extension Headers.
- *
- * About `current` and `pkt`:
- *
- *                     current     pkt
- *                     |           |
- *                     v           v
- * IPv6 <- IPv6_EXT <- IPv6_EXT <- UNDEF
- *
- * This situation may happen when the packet has a source routing extension
- * header (RFC 6554), and the packet is forwarded from an interface to another.
- *
- * @param[in] netif     The receiving interface.
- * @param[in] current   A snip to process.
- * @param[in] pkt       A packet.
- * @param[in] nh        A protocol number (see @ref net_protnum) of the current snip.
- */
-void gnrc_ipv6_demux(gnrc_netif_t *netif, gnrc_pktsnip_t *current,
-                     gnrc_pktsnip_t *pkt, uint8_t nh);
-
 /**
  * @brief   Get the IPv6 header from a given list of @ref gnrc_pktsnip_t
  *

sys/net/gnrc/network_layer/ipv6/gnrc_ipv6.c

@@ -93,8 +93,8 @@ kernel_pid_t gnrc_ipv6_init(void)
 static void _dispatch_next_header(gnrc_pktsnip_t *current, gnrc_pktsnip_t *pkt,
                                   uint8_t nh, bool interested);
 
-void gnrc_ipv6_demux(gnrc_netif_t *netif, gnrc_pktsnip_t *current,
-                     gnrc_pktsnip_t *pkt, uint8_t nh)
+static void _demux(gnrc_netif_t *netif, gnrc_pktsnip_t *current,
+                   gnrc_pktsnip_t *pkt, uint8_t nh)
 {
     bool interested;
 
@@ -848,8 +848,7 @@ static void _receive(gnrc_pktsnip_t *pkt)
 #endif /* MODULE_GNRC_IPV6_ROUTER */
     }
 
-    /* IPv6 internal demuxing (ICMPv6, Extension headers etc.) */
-    gnrc_ipv6_demux(netif, pkt, pkt, hdr->nh);
+    _demux(netif, pkt, pkt, hdr->nh);
 }
 
 /** @} */