tcp: Add num_closed_socks to struct sock_reuseport.

As noted in the following commit, a closed listener has to hold the
reference to the reuseport group for socket migration. This patch adds a
field (num_closed_socks) to struct sock_reuseport to manage closed sockets
within the same reuseport group. Moreover, this and the following commits
introduce some helper functions to split socks[] into two sections and keep
TCP_LISTEN and TCP_CLOSE sockets in each section. Like a double-ended
queue, we will place TCP_LISTEN sockets from the front and TCP_CLOSE
sockets from the end.

  TCP_LISTEN---------->       <-------TCP_CLOSE
  +---+---+  ---  +---+  ---  +---+  ---  +---+
  | 0 | 1 |  ...  | i |  ...  | j |  ...  | k |
  +---+---+  ---  +---+  ---  +---+  ---  +---+

  i = num_socks - 1
  j = max_socks - num_closed_socks
  k = max_socks - 1

This patch also extends reuseport_add_sock() and reuseport_grow() to
support num_closed_socks.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210612123224.12525-3-kuniyu@amazon.co.jp