Allow doing a snapshot of a single CPU buffer instead of a
full swap (all buffers). If this is set, then the following is
allowed:

echo 1 > /sys/kernel/debug/tracing/per_cpu/cpu2/snapshot

After which, only the tracing buffer for CPU 2 was swapped with
the main tracing buffer, and the other CPU buffers remain the same.

When this is enabled, this adds a little more overhead to the
trace recording, as it needs to add some checks to synchronize
recording with swaps. But this does not affect the performance
of the overall system. This is enabled by default when the preempt
or irq latency tracers are enabled, as those need to swap as well
and already adds the overhead (plus a lot more).