x86: fix hypercall continuation cancellation in XENMAPSPACE_gmfn_range compat wrapper

x86: fix hypercall continuation cancellation in XENMAPSPACE_gmfn_range compat wrapper

When no continuation was established, there must also not be an attempt
to cancel it - hypercall_cancel_continuation(), in the non-HVM, non-
multicall case, adjusts the guest mode return address in a way assuming
that an earlier call hypercall_create_continuation() took place.

Once touching this code, also restructure it slightly to improve
readability and switch to using the more relaxed copy function (copying
from the same guest memory already validated the virtual address
range).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>