KVM: arm64: Optimize protected mode with FWB and DIC

KVM opportunistically enables FWB if supported by the system for guest
VMs, which allows it to elude cache maintenance for data as they are
forced to be cacheable from stage-2.
In that case, __clean_dcache_guest_page() will immediately return.
However in protected mode, before calling __clean_dcache_guest_page()
it loops over the range and fix_map/unmap it, issuing TLB
invalidations, dsb() and isb() unnecessarily.

This can be optimized by returning early if FWB is supported,
kvm_pgtable_stage2_map() already issues dsb() and tlb invalidation
functions issue dsb() for the unmap path.

Similarly for DIC, we can omit the call in
invalidate_icache_guest_page()

No isb() needed as ERET is context synchronization event.

Signed-off-by: Mostafa Saleh <smostafa@google.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Link: https://patch.msgid.link/20260723162002.3848054-1-smostafa@google.com
Signed-off-by: Oliver Upton <oupton@kernel.org>

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