ANDROID: KVM: arm64: Re-instate unconditional DSB prior to TLBI

Due to a mismerge between upstream commit 7e1b2329 ("KVM: arm64:
nvhe: Synchronise with page table walker on TLBI") and pKVM commit
3b3a8600 ("ANDROID: KVM: arm64: Support TLB invalidation in guest
context"), we no longer have a DSB instruction between updating a
page-table entry and the subsequent TLB invalidation if we are already
executing in the desired stage-2 context.

Restore the missing DSB by hoisting the barrier code above the early
return logic.

Bug: 278749606
Fixes: 3b3a8600 ("ANDROID: KVM: arm64: Support TLB invalidation in guest context")
Reported-by: Peter Sewell <Peter.Sewell@cl.cam.ac.uk>
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: Ia4a2321e912e791129f38413fe0bf4d3806667b0