sysfs: add BIN_ATTR_WO() macro

This variant was missing from sysfs.h, I guess no one noticed it before.

Turns out the powerpc secure variable code can use it, so add it to the
tree for it, and potentially others to take advantage of, instead of
open-coding it.

Reported-by: Nayna Jain <nayna@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20190826150153.GD18418@kroah.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>