do_wp_page: clarify dirty_page handling

do_wp_page: clarify dirty_page handling

Reorganize the code so that dirty pages are handled closer to the place
that makes them dirty (handling write fault into shared, writable VMAs).
No behavior changes.

Signed-off-by: Michel Lespinasse <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Kosaki Motohiro <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: Theodore Tso <[email protected]>
Cc: Michael Rubin <[email protected]>
Cc: Suleiman Souhlal <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>