bridge: Linearize skb once the ND message type is validated

br_nd_send() parses ND options from ns->opt[] and therefore needs the skb
to be linear. Commit a01aee7cafc5 ("bridge: br_nd_send: linearize skb
before parsing ND options") ensured that by linearizing inside
br_nd_send() itself.

Move the linearization up into br_is_nd_neigh_msg(), right after
ndisc_check_ns_na() has validated the message as an NS/NA. This makes a
linear buffer a property of every recognized ND message, so that this and
any future ND message handling operate on a linear skb and cannot
reintroduce that class of bug by forgetting to linearize.

Since the skb is now linear by the time br_nd_send() runs, drop the
linearization there and derive ns from the transport header set by
ndisc_check_ns_na(), instead of recomputing it from the network header.

If linearization fails under memory pressure, br_is_nd_neigh_msg() returns
NULL and the packet falls back to normal forwarding rather than being
suppressed.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20260803112505.613873-5-danieller@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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