Make arch/sparc64/kernel/trampoline.S in 2.6.27.1 lock prom_entry_lock
when calling the PROM.  This prevents a race condition that I observed
causing a hang on startup on a 12-CPU E4500.

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Signed-off-by: Andrea Shepard <andrea@persephoneslair.org>
Signed-off-by: David S. Miller <davem@davemloft.net>