[AX.25]: Fix locking of ax25 protocol function list.

Delivery of AX.25 frame to the layer 3 protocols happens in softirq
context so locking needs to be bh-proof.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>