Avoid quadratic searches in segment table.

Avoid quadratic searches in segment table.

There are a couple of time that we need to iterate of the segment
table (stable) and need to drop the lock each time.
Rather than already restart after we find something of interest,
keep track of when we make a change to the table (thus possibly
breaking an iterator) and only restart when that change is seen.

Both the iterators are completely serialised so resetting
stable_changed when we retry cannot confuse a different iterator.

Signed-off-by: NeilBrown <neilb@suse.de>