mptcp: correct MPTCP_SUBFLOW_ATTR_SSN_OFFSET reserved size

ssn_offset field is u32 and is placed into the netlink response with
nla_put_u32(), but only 2 bytes are reserved for the attribute payload
in subflow_get_info_size() (even though it makes no difference
in the end, as it is aligned up to 4 bytes).  Supply the correct
argument to the relevant nla_total_size() call to make it less
confusing.

Fixes: 5147dfb5 ("mptcp: allow dumping subflow context to userspace")
Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240812065024.GA19719@asgard.redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/diag.c

@@ -94,7 +94,7 @@ static size_t subflow_get_info_size(const struct sock *sk)
 		nla_total_size(4) +	/* MPTCP_SUBFLOW_ATTR_RELWRITE_SEQ */
 		nla_total_size_64bit(8) +	/* MPTCP_SUBFLOW_ATTR_MAP_SEQ */
 		nla_total_size(4) +	/* MPTCP_SUBFLOW_ATTR_MAP_SFSEQ */
-		nla_total_size(2) +	/* MPTCP_SUBFLOW_ATTR_SSN_OFFSET */
+		nla_total_size(4) +	/* MPTCP_SUBFLOW_ATTR_SSN_OFFSET */
 		nla_total_size(2) +	/* MPTCP_SUBFLOW_ATTR_MAP_DATALEN */
 		nla_total_size(4) +	/* MPTCP_SUBFLOW_ATTR_FLAGS */
 		nla_total_size(1) +	/* MPTCP_SUBFLOW_ATTR_ID_REM */