cpuidle: haltpoll: do not shrink guest poll_limit_ns below grow_start

While adjusting guest halt poll limit, grow block starts at
guest_halt_poll_grow_start without taking intermediate values.
Similar behavior is expected while shrinking the value. This
avoids short interval values which are really not required.

VCPU1 trace (guest_halt_poll_shrink equals 2):

VCPU1 grow 10000
VCPU1 shrink 5000
VCPU1 shrink 2500
VCPU1 shrink 1250
VCPU1 shrink 625
VCPU1 shrink 312
VCPU1 shrink 156
VCPU1 shrink 78
VCPU1 shrink 39
VCPU1 shrink 19
VCPU1 shrink 9
VCPU1 shrink 4

Similar change is done in KVM halt poll flow with below patch:
Link: https://lore.kernel.org/kvm/20211006133021.271905-3-sashal@kernel.org/

Co-developed-by: Rajendran Jaishankar <jaishankar.rajendran@intel.com>
Signed-off-by: Rajendran Jaishankar <jaishankar.rajendran@intel.com>
Signed-off-by: Parshuram Sangle <parshuram.sangle@intel.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
[ rjw: Subject edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>