sctp: Constify struct sctp_sched_ops

sctp: Constify struct sctp_sched_ops

'struct sctp_sched_ops' is not modified in these drivers.

Constifying this structure moves some data to a read-only section, so
increases overall security, especially when the structure holds some
function pointers.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   8019	    568	      0	   8587	   218b	net/sctp/stream_sched_fc.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   8275	    312	      0	   8587	   218b	net/sctp/stream_sched_fc.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patch.msgid.link/dce03527eb7b7cc8a3c26d5cdac12bafe3350135.1761377890.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>