KVM: x86/mmu: Handle "default" period when selectively waking kthread

Account for the '0' being a default, "let KVM choose" period, when
determining whether or not the recovery worker needs to be awakened in
response to userspace reducing the period.  Failure to do so results in
the worker not being awakened properly, e.g. when changing the period
from '0' to any small-ish value.

Fixes: 4dfe4f40 ("kvm: x86: mmu: Make NX huge page recovery period configurable")
Cc: stable@vger.kernel.org
Cc: Junaid Shahid <junaids@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211120015706.3830341-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>