ANDROID: KVM: arm64: Introduce kvm_pgtable_stage2_reclaim_leaves

We will soon improve the mechanism by which the host's stage-2
page-table pages are recycled whenever its pool runs out of pages. To
prepare thecground for this, introduce a new helper function in the
page-table code allowing to reclaim leaf pages that don't hold counted
PTEs.

Bug: 264070847
Change-Id: Ie172bf11f2980e45bc908002368759f74f42d195
Signed-off-by: Quentin Perret <qperret@google.com>