sock_diag: Simplify sock_diag_handlers[] handling in __sock_diag_rcv_msg

The sock_diag_lock_handler() and sock_diag_unlock_handler() actually
make the code less readable. Get rid of them and make the lock usage
and access to sock_diag_handlers[] clear on the first sight.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>