sgi-xp: use lockdep_assert_held()

sgi-xp: use lockdep_assert_held()

!spin_is_locked() is always true on a UP build, so it cannot be used for
assertions.  Replace with lockdep_assert_held().

I realize UP builds are not very likely for this driver, but it's still
better to fix it.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Robin Holt <holt@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>