KVM: arm64: Fix memory leak on stage2 update of a valid PTE

When installing a new leaf PTE onto an invalid ptep, we need to
get_page(ptep) to account for the new mapping.

However, simply updating a valid PTE shouldn't result in any
additional refcounting, as there is new mapping. This otherwise
results in a page being forever wasted.

Address this by fixing-up the refcount in stage2_map_walker_try_leaf()
if the PTE was already valid, balancing out the later get_page()
in stage2_map_walk_leaf().

Signed-off-by: Yanan Wang <[email protected]>
[maz: update commit message, add comment in the code]
Signed-off-by: Marc Zyngier <[email protected]>
Acked-by: Will Deacon <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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