x86: fix spin_is_contended()

commit 7bc069c6 upstream

The masked difference is what needs to be compared against 1, rather
than the difference of masked values (which can be negative).

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>