x25: remove the BKL

This replaces all instances of lock_kernel in x25
with lock_sock, taking care to release the socket
lock around sleeping functions (sock_alloc_send_skb
and skb_recv_datagram). It is not clear whether
this is a correct solution, but it seem to be what
other protocols do in the same situation.

Includes a fix suggested by Eric Dumazet.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: David S. Miller <davem@davemloft.net>
Tested-by: Andrew Hendry <andrew.hendry@gmail.com>
Cc: linux-x25@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: Eric Dumazet <eric.dumazet@gmail.com>