scheduler_dynamic: lower the idle thread limit

Limit of 50% to mark thread as active or idle
didn't allow for multiple active threads to be placed
on single core.

Lowering the limit to 20% will allow that and force
more threads to be actively balanced.

Removing the limit was considered, but that would
cause too much thread moves when a thread with load
in single digits increased briefly. Either
due to actually doing any operation or placement
of other threads on the same core.

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: I5f8c9ff15461feb71a2d82853cfe048e412ba921
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/8289


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Community-CI: Mellanox Build Bot
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Maciej Szwed <maciej.szwed@intel.com>
Reviewed-by: Ziye Yang <ziye.yang@intel.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>