memory: do not return original value when incrementing statistics
memory: do not return original value when incrementing statistics

The function that increments statistics returns the original value,
but no one is interested in it. Simplify by dropping the return
value.

cloudius-systems/seastar,scylladb/seastar,avikivity/seastar,syuu1228/seastar,avikivity/seastar,syuu1228/seastar,cloudius-systems/seastar,syuu1228/seastar,cloudius-systems/seastar,avikivity/seastar,scylladb/seastar,scylladb/seastar