locking/osq_lock: Move the definition of optimistic_spin_node into osq_lock.c

struct optimistic_spin_node is private to the implementation.
Move it into the C file to ensure nothing is accessing it.

Signed-off-by: David Laight <david.laight@aculab.com>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>