resource: increase default reserve memory
resource: increase default reserve memory

We allowed a puny 256MB for the OS, which caused OOMs on larger machines.
Increase this to 1GB or 5% of memory, whichever is greater.  On a 128GB
machine, this is ~6GB, which should be enough for the system.

Reviewed-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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