net: add kerneldoc comment for sk_peer_lock

Fixes following warning:

include/net/sock.h:533: warning: Function parameter or member 'sk_peer_lock' not described in 'sock'

Fixes: 35306eb2

 ("af_unix: fix races in sk_peer_pid and sk_peer_cred accesses")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/r/20211001164622.58520-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>