KVM: MMU: make __kvm_mmu_free_some_pages handle empty list

(cherry picked from commit 3b80fffe)

First check if the list is empty before attempting to look at list
entries.

Signed-off-by: Izik Eidus <ieidus@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>