Fix an extent [de]allocation/[de]registration race.
Fix an extent [de]allocation/[de]registration race.

Deregister extents before deallocation, so that subsequent
reallocation/registration doesn't race with deregistration.

8l/jemalloc,8l/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,glandium/jemalloc,wqfish/jemalloc,8l/jemalloc,glandium/jemalloc,glandium/jemalloc,glandium/jemalloc,8l/jemalloc,wqfish/jemalloc