net ax25: Fix signed comparison in the sockopt handler

fixed upstream in commit b7058842 in a different way

The ax25 code tried to use

        if (optlen < sizeof(int))
                return -EINVAL;

as a security check against optlen being negative (or zero) in the
set socket option.

Unfortunately, "sizeof(int)" is an unsigned property, with the
result that the whole comparison is done in unsigned, letting
negative values slip through.

This patch changes this to

        if (optlen < (int)sizeof(int))
                return -EINVAL;

so that the comparison is done as signed, and negative values
get properly caught.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>