To replace unix_table_lock with per-hash locks in the next patch, we need
to save a hash in each socket because /proc/net/unix or BPF prog iterate
sockets while holding a hash table lock and release it later in a different
function.

Currently, we store a real/pseudo hash in struct unix_address.  However, we
do not allocate it to unbound sockets, nor should we do just for that.  For
this purpose, we can use sk_hash.  Then, we no longer use the hash field in
struct unix_address and can remove it.

Also, this patch does
  - rename unix_insert_socket() to unix_insert_unbound_socket()
  - remove the redundant list argument from __unix_insert_socket() and
     unix_insert_unbound_socket()
  - use 'unsigned int' instead of 'unsigned' in __unix_set_addr_hash()
  - remove 'inline' from unix_remove_socket() and
     unix_insert_unbound_socket().
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>