mm: refactor do_wp_page handling of shared vma into a function

mm: refactor do_wp_page handling of shared vma into a function

The do_wp_page function is extremely long.  Extract the logic for
handling a page belonging to a shared vma into a function of its own.

This helps the readability of the code, without doing any functional
change in it.

Signed-off-by: Shachar Raindel <[email protected]>
Acked-by: Linus Torvalds <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Acked-by: Andi Kleen <[email protected]>
Acked-by: Haggai Eran <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Peter Feiner <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Reviewed-by: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>