Avoid a race in lafs_get_cleanable.

Avoid a race in lafs_get_cleanable.

If we find a cleanable segment that is actually clean, then we
drop the lock and try to add it.  If something else removed it at
just this time we end up with a refcount issue as we are meant to
take references to youth_db and usage0_db when adding things to the
table, and we don't have them any more.

So simply allow the 'add_clean' to fail as that is safe.

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