ANDROID: iommu/arm-smmu-v3-kvm: Fix idmap free_leaf

As at the moment we don't support page table coalescing, it is possible
to call free_leaf while mapping a THP for a guest.
This should do nothing as the identity mapped page table doesn't
do any tracking of pages in the vmemmap.

Bug: 421734292
Change-Id: I436b9839643f8a4a76035bd2fa1e273d64456690
Signed-off-by: Mostafa Saleh <smostafa@google.com>