sctp: allow setting SCTP_SACK_IMMEDIATELY by the application

commit 27f7ed2b upstream.

This patch extends commit b93d6471 ("sctp: implement the sender side
for SACK-IMMEDIATELY extension") as it didn't white list
SCTP_SACK_IMMEDIATELY on sctp_msghdr_parse(), causing it to be
understood as an invalid flag and returning -EINVAL to the application.

Note that the actual handling of the flag is already there in
sctp_datamsg_from_user().

https://tools.ietf.org/html/rfc7053#section-7

Fixes: b93d6471 ("sctp: implement the sender side for SACK-IMMEDIATELY extension")
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[ luis: backported to 3.16:
  - dropped changes to SCTP_SNDINFO case ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

net/sctp/socket.c

@@ -6464,6 +6464,7 @@ static int sctp_msghdr_parse(const struct msghdr *msg, sctp_cmsgs_t *cmsgs)
 			/* Minimally, validate the sinfo_flags. */
 			if (cmsgs->info->sinfo_flags &
 			    ~(SCTP_UNORDERED | SCTP_ADDR_OVER |
+			      SCTP_SACK_IMMEDIATELY |
 			      SCTP_ABORT | SCTP_EOF))
 				return -EINVAL;
 			break;