Revert "Recursive lock"

Joona says that sp_update_pv() does not pass the split point
boundaries, so there is no risk to corrupt data from another
split point. Also the race on thread_should_stop() is harmless
because of this.

So revert the patch and come back to single lock.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>