KVM: arm/arm64: Change misleading use of is_error_pfn

When converting a gfn to a pfn, we call gfn_to_pfn_prot, which returns
various kinds of error values.  It turns out that is_error_pfn() only
returns true when the gfn was found in a memory slot and could somehow
not be used, but it does not return true if the gfn does not belong to
any memory slot.

Change use to is_error_noslot_pfn() which covers both cases.

Note: Since we already check for kvm_is_error_hva(hva) explicitly in the
caller of this function while holding the kvm->srcu lock protecting the
memory slots, this should never be a problem, but nevertheless this
change is warranted as it shows the intention of the code.

Reported-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>