x86/p2m: also clear defer_nested_flush on error

x86/p2m: also clear defer_nested_flush on error

While presumably benign (domain creation ought to fail when P2M setup
fails) let's be on the safe side and clear the flag as intended. And
actually, the code can be streamlined quite a bit by recognizing that
the only difference between the success and error cases is the message
printed in each case. With that, a stray spin_unlock() also goes away.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Christoph Egger <chegger@amazon.de>
Acked-by: Tim Deegan <tim@xen.org>