ipmr: fix incorrect parameter validation in the ip_mroute_getsockopt() function

The 'olr' variable can't be negative when assigned the result of
'min_t' because all 'min_t' parameters are cast to unsigned int,
and then the minimum one is chosen.

To fix the logic, check 'olr' as read from 'optlen',
where the types of relevant variables are (signed) int.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Gavrilov Ilia <Ilia.Gavrilov@infotecs.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>