net: make struct request_sock_ops::obj_size unsigned

This field is sizeof of corresponding kmem_cache so it can't be negative.

Space will be saved after 32-bit kmem_cache_create() patch.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>