ksm: three remove_rmap_item_from_tree cleanups

1. remove_rmap_item_from_tree() is called as a precaution from
   various places: don't dirty the rmap_item cacheline unnecessarily,
   just mask the flags out of the address when they have been set.

2. First get_next_rmap_item() removes an unstable rmap_item from its tree,
   then shortly afterwards cmp_and_merge_page() removes a stable rmap_item
   from its tree: it's easier just to do both at once (but definitely keep
   the BUG_ON(age > 1) which guards against a future omission).

3. When cmp_and_merge_page() moves an rmap_item from unstable to stable
   tree, it does its own rb_erase() and accounting: that's better
   expressed by remove_rmap_item_from_tree().

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>