[DECNet] fib: Fix out of bound access of dn_fib_props[]

[DECNet] fib: Fix out of bound access of dn_fib_props[]

Backported from 2.6. Found and fixed from Thomas Graf :

Fixes a typo which caused fib_props[] to have the wrong size
and makes sure the value used to index the array which is
provided by userspace via netlink is checked to avoid out of
bound access.

Signed-off-by: Thomas Graf <tgraf@suug.ch>