BUG/MEDIUM: pools/memprofile: always clean stale pool info on pool_destroy()

BUG/MEDIUM: pools/memprofile: always clean stale pool info on pool_destroy()

There's actually a problem with memprofiles: the pool pointer is stored
in ->info but some pools are replaced during startup, such as the trash
pool, leaving a dangling pointer there, that may randomly report crap or
even crash during "show profile memory".

Let's make pool_destroy() call memprof_remove_stale_info() added
by previous patch so that these entries are properly unregistered.

This must be backported along with the previous patch (MINOR:
activity/memprofile: offer a function to unregister stale info) as
far as 2.8.