include: net: add static inline dst_dev_overhead() to dst.h

[ Upstream commit 0600cf40 ]

Add static inline dst_dev_overhead() function to include/net/dst.h. This
helper function is used by ioam6_iptunnel, rpl_iptunnel and
seg6_iptunnel to get the dev's overhead based on a cache entry
(dst_entry). If the cache is empty, the default and generic value
skb->mac_len is returned. Otherwise, LL_RESERVED_SPACE() over dst's dev
is returned.

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Cc: Alexander Lobakin <aleksander.lobakin@intel.com>
Cc: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Stable-dep-of: 13e55fba ("net: ipv6: fix dst ref loop on input in rpl lwt")
Signed-off-by: Sasha Levin <sashal@kernel.org>