tcp_metrics: use netlink policy for IPv6 addr len validation

tcp_metrics: use netlink policy for IPv6 addr len validation

Use the netlink policy to validate IPv6 address length.
Destination address currently has policy for max len set,
and source has no policy validation. In both cases
the code does the real check. With correct policy
check the code can be removed.

Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20240816212245.467745-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>