ANDROID: KVM: arm64: Fix kvm_hyp_memcache::mapping leak

Upstream version of kvm_hyp_memcache adds a mapping member, allocated
during topup_hyp_memcache(). This is useful for stage-2 pages, but that
function can also be used for other memcaches e.g. to topup the hyp
allocator.

Ensure mapping stays unallocated for memcache not involved in
the stage-2 pages.

Bug: 357781595
Bug: 449003536
Change-Id: I414a6e268d6cd32fdb5d6e8d16c0d77ccd7d11d0
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>