ksmd: add general_profit metric (#3778)

Expose /sys/kernel/mm/ksm/general_profit as node_ksmd_general_profit_bytes.
This file (available since Linux 6.4, EL9) reports the net memory saved by KSM
after subtracting the overhead of rmap_item tracking structures.
Also gracefully skip any missing sysfs files instead of failing the entire
collector, since general_profit and potentially other files may not exist
on older kernels.


Assisted-by: claude-4.6-opus

Signed-off-by: Michal Skrivanek <michal.skrivanek@redhat.com>