ANDROID: KVM: arm64: Avoid switching to guest context if guest is already loaded

Typically, TLB invalidation of guest stage-2 mappings using nVHE is
performed by a hypercall originating from the host. For the invalidation
instruction to be effective, therefore, __tlb_switch_to_{guest,host}()
swizzle the active stage-2 context around the TLBI instruction.

With guest-to-host memory sharing hypercalls originating from the guest
under pKVM, there is no need to change the context when invalidating the
TLB and restoring the host context is, in fact, harmful.

Check the currently running vCPU in __tlb_switch_to_{guest,host}() and
avoid switching the context if a vCPU is already loaded.

Signed-off-by: Will Deacon <will@kernel.org>
Bug: 209580772
Change-Id: I4cfb36f0f88a2d50d50ea85a0d84e3e8191152a3
Signed-off-by: Will Deacon <willdeacon@google.com>