ANDROID: KVM: arm64: Fix MMU context save/restore over TLB invalidation

The 'mmu' parameter to enter_vmid_context() represents the target MMU
to switch to, so we should stash away the current MMU for restoration
by exit_vmid_context() rather than the one we're about to switch to!

Bug: 291568386
Fixes: 4c192954 ("ANDROID: KVM: arm64: Support TLB invalidation in guest context")
Tested-by: Mostafa Saleh <smostafa@google.com>
Reported-by: Mostafa Saleh <smostafa@google.com>
[willdeacon@: Rename 'host_s2_mmu' to 'host_mmu']
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I5d76c159424e32a6d70c598d0007f98ea80c1db4