net: core: annotate socks of struct sock_reuseport with __counted_by

According to '__reuseport_alloc()', annotate flexible array member
'sock' of 'struct sock_reuseport' with '__counted_by()' and use
convenient 'struct_size()' to simplify the math used in 'kzalloc()'.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20240801142311.42837-1-dmantipov@yandex.ru


Signed-off-by: Jakub Kicinski <kuba@kernel.org>