KVM: arm64: Fix uninitialized memcache pointer in user_mem_abort()

In the Linux kernel, the following vulnerability has been resolved:

KVM: arm64: Fix uninitialized memcache pointer in user_mem_abort()

Commit fce886a60207 ("KVM: arm64: Plumb the pKVM MMU in KVM") made the

initialization of the local memcache variable in user_mem_abort()

conditional, leaving a codepath where it is used uninitialized via

kvm_pgtable_stage2_map().

This can fail on any path that requires a stage-2 allocation

without transition via a permission fault or dirty logging.

Fix this by making sure that memcache is always valid.