net/x25: push BKL usage into x25_proto

The x25 driver uses lock_kernel() implicitly through
its proto_ops wrapper. The makes the usage explicit
in order to get rid of that wrapper and to better document
the usage of the BKL.

The next step should be to get rid of the usage of the BKL
in x25 entirely, which requires understanding what data
structures need serialized accesses.

Cc: Henner Eisen <eis@baty.hanse.de>
Cc: David S. Miller <davem@davemloft.net>
Cc: linux-x25@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>