ANDROID: KVM: arm64: Pin host stage-2 tables

Now that the host stage-2 page-table is entirely pre-populated in
__pkvm_init_finalize(), we know that by the end of this function, the
structure of the page-table will remain stable until the host calls in
the hypervisor to require e.g. a page-table changes (by e.g. running a
guest). This does not necessarily mean that no host mem aborts will
occur -- there may be null PTEs in the host stage-2 due to collapsed
block mappings from fix_host_ownership() for example -- but all those
aborts should be trivially handled without requiring structural changes
to the page-table. This has the nice side effect of guaranteeing that
host_mem_abort() will not allocate from the host stage-2 pool. In order
to ensure this desirable property is retained for the lifetime of the
system even in the presence of the coalescing feature, let's 'pin' the
structure of the page-table as-is by taking an additional reference
from each table entry.

Bug: 264070847
Change-Id: If870d7485cc38f6ad714901e710287911f111897
Signed-off-by: Quentin Perret <qperret@google.com>