ANDROID: KVM: arm64: Check for existing pinned_pages before THP in pkvm_mem_abort

With the later introduction of the SPLIT request, we can expect to have
a level-3 mapping on the stage-2 where a level-2 is installed for the
stage-1. Also, part of the stage-2 might have been unmapped already. So
check for existing stage-2 mappings before checking for the stage-1
(transparent_hugepage_adjust()).

Bug: 419548963
Bug: 357781595
Bug: 278011447
Change-Id: Iff7755ff6a82fa080c3a04efe1e689bc0500a3c9
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>