[AX.25]: Fix potencial memory hole.

If the AX.25 dialect chosen by the sysadmin is set to DAMA master / 3
(or DAMA slave / 2, if CONFIG_AX25_DAMA_SLAVE=n) ax25_kick() will fall
through the switch statement without calling ax25_send_iframe() or any
other function that would eventually free skbn thus leaking the packet.

Fix by restricting the sysctl inferface to allow only actually supported
AX.25 dialects.

The system administration mistake needed for this to happen is rather
unlikely, so this is an uncritical hole.

Coverity #651.

Signed-off-by: Ralf Baechle DL5RB <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>