net: inet: Open code inet_hash2 and inet_unhash2

This patch folds lhash2 related functions into __inet_hash and
inet_unhash.  This will make the removal of the listening_hash
in a latter patch easier to review.

First, this patch folds inet_hash2 into __inet_hash.

For unhash, the current call sequence is like
inet_unhash() => __inet_unhash() => inet_unhash2().
The specific testing cases in __inet_unhash() are mostly related
to TCP_LISTEN sk and its caller inet_unhash() already has
the TCP_LISTEN test, so this patch folds both __inet_unhash() and
inet_unhash2() into inet_unhash().

Note that all listening_hash users also have lhash2 initialized,
so the !h->lhash2 check is no longer needed.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>