powerpc/nvram: Add spinlock to oops_to_nvram to prevent oops in compression code.

When issuing a system reset we almost always oops in the oops_to_nvram
code because multiple CPUs are using the deflate work area. Add a
spinlock to protect it.

To play it safe I'm using trylock to avoid locking up if the NVRAM
code oopses. This means we might miss multiple CPUs oopsing at exactly
the same time but I think it's best to play it safe for now. Once we
are happy with the reliability we can change it to a full spinlock.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Jim Keniston <jkenisto@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>