sctp: handle the error returned from sctp_auth_asoc_init_active_key

In the Linux kernel, the following vulnerability has been resolved:

sctp: handle the error returned from sctp_auth_asoc_init_active_key

When it returns an error from sctp_auth_asoc_init_active_key(), the
active_key is actually not updated. The old sh_key will be freeed
while it's still used as active key in asoc. Then an use-after-free
will be triggered when sending patckets, as found by syzbot:

sctp_auth_shkey_hold+0x22/0xa0 net/sctp/auth.c:112
sctp_set_owner_w net/sctp/socket.c:132 [inline]
sctp_sendmsg_to_asoc+0xbd5/0x1a20 net/sctp/socket.c:1863
sctp_sendmsg+0x1053/0x1d50 net/sctp/socket.c:2025
inet_sendmsg+0x99/0xe0 net/ipv4/af_inet.c:819
sock_sendmsg_nosec net/socket.c:714 [inline]
sock_sendmsg+0xcf/0x120 net/socket.c:734

This patch is to fix it by not replacing the sh_key when it returns
errors from sctp_auth_asoc_init_active_key() in sctp_auth_set_key().
For sctp_auth_set_active_key(), old active_key_id will be set back
to asoc->active_key_id when the same thing happens.