This re-implements the nonintrusive spin-polling loop for the
SMP+PREEMPT spinlock/rwlock variants, using the new *_can_lock()
primitives.  (The patch also adds *_can_lock() to the UP branch of
spinlock.h, for completeness.)

build- and boot-tested on x86 SMP+PREEMPT and SMP+!PREEMPT.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>