bridge: Validate NS/NA messages using ndisc_check_ns_na()

The bridge performs neighbor suppression by snooping NS/NA messages, but
previously only checked the ICMPv6 type and code. This leaves it open to
acting on malformed or spoofed packets that any RFC-compliant node should
reject.

Wire br_is_nd_neigh_msg() into the new ndisc_check_ns_na() helper, which
enforces the full RFC 4861 section 7.1.1/7.1.2 receive validation:
hop limit of 255, valid checksum, correct code, and type-specific rules
(NS target not multicast; NA solicited flag clear for multicast
destinations).

MLD messages are already validated by ipv6_mc_check_mld() before the
bridge acts on them; this brings NS/NA to the same standard.

As a side effect, the skb parameter of br_is_nd_neigh_msg() changes from
const to non-const, since ndisc_check_ns_na() may reallocate the skb head
via pskb_may_pull() and sets the transport header. The returned pointer is
now derived from skb_transport_header() rather than a direct cast.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Link: https://patch.msgid.link/20260803112505.613873-4-danieller@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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