[IA64] Fix spinaphore down_spin()

Typo in down_spin() meant it only read the low 32 bits of the
"serve" value, instead of the full 64 bits. This results in the
system hanging when the values in ticket/serve get larger than
32-bits. A big enough system running the right test can hit this
in a just a few hours.

Broken since 883a3acf
    [IA64] Re-implement spinaphores using ticket lock concepts

Reported via IRC by Bjorn Helgaas

Signed-off-by: Tony Luck <tony.luck@intel.com>