mm: optimize pfn calculation in online_page()

mm: optimize pfn calculation in online_page()

If CONFIG_FLATMEM is enabled pfn is calculated in online_page() more than
once.  It is possible to optimize that and use value established at
beginning of that function.

Signed-off-by: Daniel Kiper <[email protected]>
Acked-by: Dave Hansen <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Wu Fengguang <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Christoph Lameter <[email protected]>
Acked-by: David Rientjes <[email protected]>
Reviewed-by: Jesper Juhl <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>