mptcp: add csum_reqd in mptcp_out_options

This patch added a new member csum_reqd in struct mptcp_out_options and
struct mptcp_subflow_request_sock. Initialized it with the helper
function mptcp_is_checksum_enabled().

In mptcp_write_options, if this field is enabled, send out the MP_CAPABLE
suboption with the MPTCP_CAP_CHECKSUM_REQD flag.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/mptcp.h b/include/net/mptcp.h
index 23bbd43..33af68e 100644
--- a/include/net/mptcp.h
+++ b/include/net/mptcp.h
@@ -64,8 +64,9 @@ struct mptcp_out_options {
 	struct mptcp_rm_list rm_list;
 	u8 join_id;
 	u8 backup;
-	u8 reset_reason:4;
-	u8 reset_transient:1;
+	u8 reset_reason:4,
+	   reset_transient:1,
+	   csum_reqd:1;
 	u32 nonce;
 	u64 thmac;
 	u32 token;