ksm: don't fail stable tree lookups if walking over stale stable_nodes

The stable_nodes can become stale at any time if the underlying pages gets
freed.  The stable_node gets collected and removed from the stable rbtree
if that is detected during the rbtree lookups.

Don't fail the lookup if running into stale stable_nodes, just restart the
lookup after collecting the stale stable_nodes.  Otherwise the CPU spent
in the preparation stage is wasted and the lookup must be repeated at the
next loop potentially failing a second time in a second stale stable_node.

If we don't prune aggressively we delay the merging of the unstable node
candidates and at the same time we delay the freeing of the stale
stable_nodes.  Keeping stale stable_nodes around wastes memory and it
can't provide any benefit.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Petr Holasek <pholasek@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>