lib/mempool: One more spurious -ENOMEM condition

Similar to the last patch, there was a spot in block recombination
where the lock would be released while the combined block was being
held allocated.  That means that when recombining a single top-level
block, it was possible for the entire heap to look allocated.

Make the combination and re-addition of the larger block atomic.
Requires a little surgery to the structure of the code, so this is a
little more involved than the earlier fix.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>