sctp: Stop accepting md5 and sha1 for net.sctp.cookie_hmac_alg

The upgrade of the cookie authentication algorithm to HMAC-SHA256 kept
some backwards compatibility for the net.sctp.cookie_hmac_alg sysctl by
still accepting the values 'md5' and 'sha1'. Those algorithms are no
longer actually used, but rather those values were just treated as
requests to enable cookie authentication.

As requested at
https://lore.kernel.org/netdev/CADvbK_fmCRARc8VznH8cQa-QKaCOQZ6yFbF=1-VDK=[e-mail]/
and https://lore.kernel.org/netdev/[e-mail]/ ,
go further and start rejecting 'md5' and 'sha1' completely.

Signed-off-by: Eric Biggers <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/ip-sysctl.rst

···
 	* sha256
 	* none
 
-	md5 and sha1 are also accepted for backwards compatibility, but cause
-	sha256 to be selected.
-
 	Default: sha256
 
 rcvbuf_policy - INTEGER

net/sctp/sysctl.c

···
 		ret = proc_dostring(&tbl, 1, buffer, lenp, ppos);
 		if (ret)
 			return ret;
-		if (!strcmp(tmp, "sha256") ||
-		    /* for backwards compatibility */
-		    !strcmp(tmp, "md5") || !strcmp(tmp, "sha1")) {
+		if (!strcmp(tmp, "sha256")) {
 			net->sctp.cookie_auth_enable = 1;
 			return 0;
 		}