scylla_swap_setup: limit swapfile size to half of diskspace
scylla_swap_setup: limit swapfile size to half of diskspace

We should not fill entire disk space with swapfile, it's safer to limit
swap size 50% of diskspace.
Also, if 50% of diskspace <= 1GB, abort setup since it's out of disk space.

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