ftrace: Optimize testing what context current is in

The preempt_count() is not a simple location in memory, it could be part of
per_cpu code or more. Each access to preempt_count(), or one of its accessor
functions (like in_interrupt()) takes several cycles. By reading
preempt_count() once, and then doing tests to find the context against the
value return is slightly faster than using in_nmi() and in_interrupt().

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]

Signed-off-by: Steven Rostedt (VMware) <[email protected]>

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