ANDROID: KVM: arm64: Support TLB invalidation in guest context

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 and unsharing hypercalls originating
from the guest under pKVM, there is now a need to support both guest
and host VMID invalidations issued from guest context.

Replace the __tlb_switch_to_{guest,host}() functions with a more general
{enter,exit}_vmid_context() implementation which supports being invoked
from guest context and acts as a no-op if the target context matches the
running context.

Bug: 209580772
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I259b1acd11f25143080af2deaab820746090436d