netlink: specs: tcp_metrics: fix the attribute length checks

TCP_FASTOPEN_COOKIE_MAX is the longest cookie we accept, not the shortest
one. Cookies are even sized, from 4 bytes up, and the ones Linux itself
generates are 8 bytes, so a 16 byte minimum declares all but the longest
cookie invalid. The reference policy kept under #if 0 in tcp_metrics.c
spells it as a maximum, which is what .len means for NLA_BINARY.

The IPv6 addresses err the other way. The policy uses
NLA_POLICY_EXACT_LEN() for both, so a request carrying a longer address is
rejected with -ERANGE, even though the spec advertises 16 bytes as a mere
minimum. Were the policy generated from this spec, as kernel-policy: global
promises, the check would turn into NLA_POLICY_MIN_LEN() and start
accepting over-long addresses, of which nla_get_in6_addr() would take the
first 16 bytes.

Nothing generated changes. fopen-cookie is reply-only so it never gets a
policy entry, and exact-len only feeds the policy (and the fixed size array
form, which needs a sub-type).

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>
Link: https://patch.msgid.link/20260912234337.308298-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/netlink/specs/tcp_metrics.yaml

@@ -32,7 +32,7 @@ attribute-sets:
         name: addr-ipv6
         type: binary
         checks:
-          min-len: 16
+          exact-len: 16
         byte-order: big-endian
         display-hint: ipv6
       -
@@ -63,7 +63,7 @@ attribute-sets:
         name: fopen-cookie
         type: binary
         checks:
-          min-len: tcp-fastopen-cookie-max
+          max-len: tcp-fastopen-cookie-max
       -
         name: saddr-ipv4
         type: u32
@@ -73,7 +73,7 @@ attribute-sets:
         name: saddr-ipv6
         type: binary
         checks:
-          min-len: 16
+          exact-len: 16
         byte-order: big-endian
         display-hint: ipv6
       -