ratelimit: Fix/allow use in atomic contexts

I'd like to use printk_ratelimit() in NMI context, but it's not
robust right now due to spinlock usage in lib/ratelimit.c. If an
NMI is unlucky enough to hit just that spot we might lock up trying
to take the spinlock again.

Fix that by using a trylock variant. If we contend on that lock we
can genuinely skip the message because the state is just being
accessed by another CPU (or by this CPU).

( We could use atomics for the suppressed messages field, but
  i doubt it matters in practice and it makes the code heavier. )

Cc: Peter Zijlstra <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: David S. Miller <[email protected]>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <[email protected]>

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