KVM: arm64: Infer PA offset from VA in hyp map walker

Similar to the recently fixed stage-2 walker, the hyp map walker
increments the PA and VA of a walk separately. Unlike stage-2, there is
no bug here as the map walker has exclusive access to the stage-1 page
tables.

Nonetheless, in the interest of continuity throughout the page table
code, tweak the hyp map walker to avoid incrementing the PA and instead
use the VA as the authoritative source of how far along a table walk has
gotten. Calculate the PA to use for a leaf PTE by adding the offset of
the VA from the start of the walk to the starting PA.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230421071606.1603916-3-oliver.upton@linux.dev