locking/osq_lock: Clarify osq_wait_next() calling convention

osq_wait_next() is passed 'prev' from osq_lock() and NULL from
osq_unlock() but only needs the 'cpu' value to write to lock->tail.

Just pass prev->cpu or OSQ_UNLOCKED_VAL instead.

Should have no effect on the generated code since gcc manages to assume
that 'prev != NULL' due to an earlier dereference.

Signed-off-by: David Laight <david.laight@aculab.com>
[ Changed 'old' to 'old_cpu' by request from Waiman Long  - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>