powerpc/qspinlock: allow new waiters to steal the lock before queueing

Allow new waiters to "steal" the lock before queueing. That is, to
acquire it while other CPUs have queued.

This particularly helps paravirt performance when physical CPUs are
oversubscribed, by keeping the lock from becoming a strict FIFO and
vCPU preemption causing queue train wrecks.

The new __queued_spin_trylock_steal() function is put in qspinlock.h
to save having to move it, because it will be used there by a later
change.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221126095932.1234527-5-npiggin@gmail.com

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