backfire: [kernel] swconfig: Check vlan/port indexes for validity. (backport of r20811)

backfire: [kernel] swconfig: Check vlan/port indexes for validity. (backport of r20811)

Swconfig needs to make sure that requested vlans/ports actually exist,
else it might read or modify memory not belonging to itself.

This patch adds a quick range check in swconfig's kernel part to
prevent accidential or intentional memory modification.

Signed-off-by: Jonas Gorski <jonas.gorski+openwrt@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/branches/backfire@21846 3c298f89-4303-0410-b956-a3cf2f4a3e73