powerpc/qspinlock: add ability to prod new queue head CPU

After the head of the queue acquires the lock, it releases the
next waiter in the queue to become the new head. Add an option
to prod the new head if its vCPU was preempted. This may only
have an effect if queue waiters are yielding.

Disable this option by default for now, i.e., no logical change.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221126095932.1234527-12-npiggin@gmail.com