tcp: use __jhash_final() in inet6_ehashfn()

I misread jhash2() implementation.

Last round should use __jhash_final() instead of __jhash_mix().

Using __jhash_mix() here leaves entropy distributed across a, b, and c,
which might lead to incomplete diffusion of the faddr and fport bits
into the bucket index. Replacing this last __jhash_mix() with
__jhash_final() provides the correct avalanche properties
for the returned value in c.

$ scripts/bloat-o-meter -t vmlinux.0 vmlinux
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-4 (-4)
Function                                     old     new   delta
inet6_ehashfn                                306     302      -4
Total: Before=25155089, After=25155085, chg -0.00%

Fixes: 854587e69e ("tcp: improve inet6_ehashfn() entropy")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260327040646.3849503-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/inet6_hashtables.c

@@ -42,7 +42,7 @@ u32 inet6_ehashfn(const struct net *net,
 	 * Hash laddr + faddr + lport/fport + net_hash_mix.
 	 * Notes:
 	 * We combine laddr[0] (high order 32 bits of local address)
-	 * with net_hash_mix() to avoid using __jhash_final(a, b, c).
+	 * with net_hash_mix() to hash a multiple of 3 words.
 	 *
 	 * We do not include JHASH_INITVAL + 36 contribution
 	 * to initial values of a, b, c.
@@ -63,7 +63,7 @@ u32 inet6_ehashfn(const struct net *net,
 	a += (__force u32)faddr->s6_addr32[2];
 	b += (__force u32)faddr->s6_addr32[3];
 	c += (__force u32)fport;
-	__jhash_mix(a, b, c);
+	__jhash_final(a, b, c);
 
 	/* Note: We need to add @lport instead of fully hashing it.
 	 * See commits 9544d60a2605 ("inet: change lport contribution