seg6: Fix validation of nexthop addresses

seg6: Fix validation of nexthop addresses

The kernel currently validates that the length of the provided nexthop
address does not exceed the specified length. This can lead to the
kernel reading uninitialized memory if user space provided a shorter
length than the specified one.

Fix by validating that the provided length exactly matches the specified
one.

Fixes: d1df6fd8a1d2 ("ipv6: sr: define core operations for seg6local lightweight tunnel")
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250604113252.371528-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>