Merge branch 'bpf: Batching iter for AF_UNIX sockets.'

Kuniyuki Iwashima says:

====================

Last year the commit afd20b92 ("af_unix: Replace the big lock with
small locks.") landed on bpf-next.  Now we can use a batching algorithm
for AF_UNIX bpf iter as TCP bpf iter.

Changelog:
- Add the 1st patch.
- Call unix_get_first() in .start()/.next() to always acquire a lock in
  each iteration in the 2nd patch.
====================

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