Use different constraint for gcc < 4.1 in bitops.h

+m is really correct for a RMW instruction, but some older gccs
error out. I finally gave in and ifdefed it.

This fixes compilation errors with some compiler version.
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>