x86: Use correct byte-sized register constraint in __xchg_op()

x86-64 can access the low half of any register, but i386 can only do
it with a subset of registers.  'r' causes compilation failures on i386,
but 'q' expresses the constraint properly.

Signed-off-by: Jeremy Fitzhardinge <jeremy@goop.org>
Link: http://lkml.kernel.org/r/4F7A3315.501@goop.org
Reported-by: Leigh Scott <leigh123linux@googlemail.com>
Tested-by: Thomas Reitmayr <treitmayr@devbase.at>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: <stable@vger.kernel.org> v3.3