xen: fix XEN_DOMCTL_gdbsx_guestmemio crash

xen: fix XEN_DOMCTL_gdbsx_guestmemio crash

A hypervisor built without CONFIG_GDBSX will crash in case the
XEN_DOMCTL_gdbsx_guestmemio domctl is being called, as the call will
end up in iommu_do_domctl() with d == NULL:

  (XEN) CPU:    6
  (XEN) RIP:    e008:[<ffff82d040269984>] iommu_do_domctl+0x4/0x30
  (XEN) RFLAGS: 0000000000010202   CONTEXT: hypervisor (d0v0)
  (XEN) rax: 00000000000003e8   rbx: ffff830856277ef8   rcx: ffff830856277fff
  ...
  (XEN) Xen call trace:
  (XEN)    [<ffff82d040269984>] R iommu_do_domctl+0x4/0x30
  (XEN)    [<ffff82d04035cd5f>] S arch_do_domctl+0x7f/0x2330
  (XEN)    [<ffff82d040239e46>] S do_domctl+0xe56/0x1930
  (XEN)    [<ffff82d040238ff0>] S do_domctl+0/0x1930
  (XEN)    [<ffff82d0402f8c59>] S pv_hypercall+0x99/0x110
  (XEN)    [<ffff82d0402f5161>] S arch/x86/pv/domain.c#_toggle_guest_pt+0x11/0x90
  (XEN)    [<ffff82d040366288>] S lstar_enter+0x128/0x130
  (XEN)
  (XEN) Pagetable walk from 0000000000000144:
  (XEN)  L4[0x000] = 0000000000000000 ffffffffffffffff
  (XEN)
  (XEN) ****************************************
  (XEN) Panic on CPU 6:
  (XEN) FATAL PAGE FAULT
  (XEN) [error_code=0000]
  (XEN) Faulting linear address: 0000000000000144
  (XEN) ****************************************

It used to be permitted to pass DOMID_IDLE to dbg_rw_mem(), which is why the
special case skipping the domid checks exists.  Now that it is only permitted
to pass proper domids, remove the special case, making 'd' always valid.

Reported-by: Cheyenne Wills <cheyenne.wills@gmail.com>
Fixes: e726a82ca0dc ("xen: make gdbsx support configurable")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>