HWPOISON: Add PROC_FS dependency to hwpoison injector v2

The injector filter requires stable_page_flags() which is supplied
by procfs. So make it dependent on that.

Also add ifdefs around the filter code in memory-failure.c so that
when the filter is disabled due to missing dependencies the whole
code still builds.

Reported-by: Ingo Molnar
Signed-off-by: Andi Kleen <ak@linux.intel.com>