fuse: remove FUSE_NOTIFY_CODE_MAX from <uapi/linux/fuse.h>

Constants that change value from version to version have no place in an
interface definition.

Hopefully this won't break anything.

Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>