memory: use size instead of old_size to shrink memory block
memory: use size instead of old_size to shrink memory block

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

dreamsxin/seastar,kjniemi/seastar,syuu1228/seastar,scylladb/scylla-seastar,cloudius-systems/seastar,scylladb/seastar,cloudius-systems/seastar,cloudius-systems/seastar,avikivity/seastar,scylladb/seastar,scylladb/seastar,kjniemi/seastar,sjperkins/seastar,scylladb/scylla-seastar,kjniemi/seastar,dreamsxin/seastar,syuu1228/seastar,sjperkins/seastar,scylladb/scylla-seastar,sjperkins/seastar,syuu1228/seastar,avikivity/seastar,dreamsxin/seastar,avikivity/seastar