ksmbd: Constify struct ksmbd_transport_ops

'struct ksmbd_transport_ops' is not modified in these drivers.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  52184	   2085	    256	  54525	   d4fd	fs/smb/server/transport_rdma.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  52260	   2021	    256	  54537	   d509	fs/smb/server/transport_rdma.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>