KVM: s390: Refactor kvm_is_error_gpa() into kvm_is_gpa_in_memslot()

KVM: s390: Refactor kvm_is_error_gpa() into kvm_is_gpa_in_memslot()

Rename kvm_is_error_gpa() to kvm_is_gpa_in_memslot() and invert the
polarity accordingly in order to (a) free up kvm_is_error_gpa() to match
with kvm_is_error_{hva,page}(), and (b) to make it more obvious that the
helper is doing a memslot lookup, i.e. not simply checking for INVALID_GPA.

No functional change intended.

Link: https://lore.kernel.org/r/20240215152916.1158-9-paul@xen.org
Signed-off-by: Sean Christopherson <seanjc@google.com>