linuxkpi/sysfs.h: Cast arg2 through intptr_t to avoid GCC warning

The code compiles fine under Clang, but GCC on PPC is less permissive about
integer and pointer sizes. (An intmax_t is clearly *large enough* to hold a
pointer value.)

Another follow-up to r290475.

Reported by: jhibbits
Sponsored by: EMC / Isilon Storage Division