firmware: google: Test spinlock on panic path to avoid lockups

Currently the gsmi driver registers a panic notifier as well as
reboot and die notifiers. The callbacks registered are called in
atomic and very limited context - for instance, panic disables
preemption and local IRQs, also all secondary CPUs (not executing
the panic path) are shutdown.

With that said, taking a spinlock in this scenario is a dangerous
invitation for lockup scenarios. So, fix that by checking if the
spinlock is free to acquire in the panic notifier callback - if not,
bail-out and avoid a potential hang.

Fixes: 74c5b31c6618 ("driver: Google EFI SMI")
Cc: Andrew Morton <[email protected]>
Cc: Ard Biesheuvel <[email protected]>
Cc: David Gow <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Julius Werner <[email protected]>
Cc: Petr Mladek <[email protected]>
Reviewed-by: Evan Green <[email protected]>
Signed-off-by: Guilherme G. Piccoli <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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