mm: generalize HUGETLB_PAGE_SIZE_VARIABLE

HUGETLB_PAGE_SIZE_VARIABLE need not be defined for each individual
platform subscribing it.  Instead just make it generic.

Link: https://lkml.kernel.org/r/1614914928-22039-1-git-send-email-anshuman.khandual@arm.com


Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>	[powerpc]
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>