mm: introduce page_size()

mm: introduce page_size()

Patch series "Make working with compound pages easier", v2.

These three patches add three helpers and convert the appropriate
places to use them.

This patch (of 3):

It's unnecessarily hard to find out the size of a potentially huge page.
Replace 'PAGE_SIZE << compound_order(page)' with page_size(page).

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Reviewed-by: Ira Weiny <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>