af_unix: Refactor unix_next_socket().

Currently, unix_next_socket() is overloaded depending on the 2nd argument.
If it is NULL, unix_next_socket() returns the first socket in the hash.  If
not NULL, it returns the next socket in the same hash list or the first
socket in the next non-empty hash list.

This patch refactors unix_next_socket() into two functions unix_get_first()
and unix_get_next().  unix_get_first() newly acquires a lock and returns
the first socket in the list.  unix_get_next() returns the next socket in a
list or releases a lock and falls back to unix_get_first().

In the following patch, bpf iter holds entire sockets in a list and always
releases the lock before .show().  It always calls unix_get_first() to
acquire a lock in each iteration.  So, this patch makes the change easier
to follow.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Link: https://lore.kernel.org/r/20220113002849.4384-2-kuniyu@amazon.co.jp


Signed-off-by: Alexei Starovoitov <ast@kernel.org>