ci_lasttb should be u_quad_t?

[Aymeric Vincent <Aymeric.Vincent%labri.fr@localhost>]

While investigating the lockups on macppc, I noticed in
sys/arch/powerpc/include/cpu.h
that the ci_lasttb field of struct cpu_info is a u_long.

However, seeing how it is used in sys/arch/macppc/macppc/clock.c
in microtime(), I suspect it should really contain the full tb and not
just its lower bits.

Is there a reason that I'm not seeing for keeping only the low-order
bits? Or is it just that microtime() is not an important function and
it can be expected to produce values going backward "very rarely"?

 Aymeric