cifs: fix hardcoded default security descriptor length

It was hardcoded to 192 bytes, which was not enough when the max number
of subauthorities went to 15. Redefine this constant in terms of sizeof
the structs involved, and rename it for better clarity.

While we're at it, remove a couple more unused constants from cifsacl.h.

Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>