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

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=zRqv_cXw@mail.gmail.com/
and https://lore.kernel.org/netdev/20250818084345.708ac796@kernel.org/ ,
go further and start rejecting 'md5' and 'sha1' completely.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://patch.msgid.link/20250818205426.30222-6-ebiggers@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>