x86: refine guest_mode()

x86: refine guest_mode()

The 2nd of the assertions as well as the macro's return value have been
assuming we're on the primary stack. While for most IST exceptions we
switch back to the main one when user mode was interrupted, for #DF we
intentionally never do, and hence a #DF actually triggering on a user
mode insn (which then is still a Xen bug) would in turn trigger this
assertion, rather than cleanly logging state.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>