Because they can be, and because code like this produces a warning if
they're not:

struct device_attribute dev_attr;

sysfs_notify(&kobj, NULL, dev_attr.attr.name);
Signed-off-by: NTrent Piepho <tpiepho@freescale.com>
CC: Neil Brown <neilb@suse.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>