ipv4: ipmr: various fixes and cleanups

1) ip_mroute_setsockopt() & ip_mroute_getsockopt() should not
   access/set raw_sk(sk)->ipmr_table before making sure the socket
   is a raw socket, and protocol is IGMP

2) MRT_INIT should return -EINVAL if optlen != sizeof(int), not
   -ENOPROTOOPT

3) MRT_ASSERT & MRT_PIM should validate optlen

4) " (v) ? 1 : 0 " can be written as " !!v "

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>