kexec: save PG_head_mask in VMCOREINFO

To allow filtering of huge pages, makedumpfile must be able to identify
them in the dump. This can be done by checking the appropriate page
flag, so communicate its value to makedumpfile through the VMCOREINFO
interface.

There's only one small catch. Depending on how many page flags are
available on a given architecture, this bit can be called PG_head or
PG_compound.

I sent a similar patch back in 2012, but Eric Biederman did not like
using an #ifdef. So, this time I'm adding a common symbol
(PG_head_mask) instead.

See https://lkml.org/lkml/2012/11/28/91 for the previous version.

Signed-off-by: Petr Tesarik <[e-mail]>
Acked-by: Vivek Goyal <[e-mail]>
Cc: Eric Biederman <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
Cc: Fengguang Wu <[e-mail]>
Cc: Benjamin Herrenschmidt <[e-mail]>
Cc: Shaohua Li <[e-mail]>
Cc: Alexey Kardashevskiy <[e-mail]>
Cc: Sasha Levin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/page-flags.h

···
 	ClearPageHead(page);
 }
 #endif
+
+#define PG_head_mask ((1L << PG_head))
+
 #else
 /*
  * Reduce page flag use as much as possible by overlapping

kernel/kexec.c

···
 #ifdef CONFIG_MEMORY_FAILURE
 	VMCOREINFO_NUMBER(PG_hwpoison);
 #endif
+	VMCOREINFO_NUMBER(PG_head_mask);
 	VMCOREINFO_NUMBER(PAGE_BUDDY_MAPCOUNT_VALUE);
 
 	arch_crash_save_vmcoreinfo();