bpf: Constify struct btf_kind_operations

struct btf_kind_operations are not modified in BTF.

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

On a x86_64, with allmodconfig:

Before:
======
   text	   data	    bss	    dec	    hex	filename
 184320	   7091	    548	 191959	  2edd7	kernel/bpf/btf.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
 184896	   6515	    548	 191959	  2edd7	kernel/bpf/btf.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/9192ab72b2e9c66aefd6520f359a20297186327f.1726417289.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Alexei Starovoitov <ast@kernel.org>