mm: nominate faultaround area in bytes rather than page order

There is evidencs that the faultaround feature is less relevant on
architectures with page size bigger then 4k.  Which makes sense since page
fault overhead per byte of mapped area should be less there.

Let's rework the feature to specify faultaround area in bytes instead of
page order.  It's 64 kilobytes for now.

The patch effectively disables faultaround on architectures with page size
>= 64k (like ppc64).

It's possible that some other size of faultaround area is relevant for a
platform.  We can expose `fault_around_bytes' variable to arch-specific
code once such platforms will be found.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Hugh Dickins <hughd@google.com>
Cc: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>