gdb: Fix small pool memory usage reporting in 'scylla memory'
gdb: Fix small pool memory usage reporting in 'scylla memory'

Uses span_checker to work around for corrupted _pages_in_use.

Refs https://github.com/scylladb/seastar/issues/608

As a bonus, calculates use_count correctly for fallback spans.

avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla