[AX.25]: Optimize AX.25 socket list lock

Right now all uses of the ax25_list_lock lock are _bh locks but knowing
some code is only ever getting invoked from _bh context we can better.

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