mm/rmap: Convert the struct anon_vma::mutex to an rwsem

mm/rmap: Convert the struct anon_vma::mutex to an rwsem

Convert the struct anon_vma::mutex to an rwsem, which will help
in solving a page-migration scalability problem. (Addressed in
a separate patch.)

The conversion is simple and straightforward: in every case
where we mutex_lock()ed we'll now down_write().

Suggested-by: Linus Torvalds <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: Lee Schermerhorn <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Hugh Dickins <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Signed-off-by: Mel Gorman <[email protected]>