linuxkpi: Fix __ATTR_RO() in <linux/sysfs.h>

[Why]
The passed structure may not have a .store field. This is the case in
the amdgpu DRM driver starting with Linux 5.18.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D43023

(cherry picked from commit d752a5e82ac92603fadf82e7de75878e1f5d054a)