sctp: enlarge SO_RCVBUF option value

When using setsockopt() with recent Linux kernels, it doesn't make sense to
use a value smaller than SOCK_MIN_RCVBUF/2 for SO_RCVBUF and expect getting
a SO_RCVBUF value by getsockopt() which is doubled.  See the following code
in linux/net/core/sock.c:

	if ((val * 2) < SOCK_MIN_RCVBUF)
		sk->sk_rcvbuf = SOCK_MIN_RCVBUF;
	else
		sk->sk_rcvbuf = val * 2;
	break;

SOCK_MIN_RCVBUF is defined as (2048 + sizeof(struct sk_buff)) which is 2240.

Signed-off-by: Simon Xu <xu.simon@oracle.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>
Reviewed-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>