perf: Fix get_recursion_context()

One should use in_serving_softirq() to detect SoftIRQ context.

Fixes: 96f6d444 ("perf_counter: avoid recursion")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201030151955.120572175@infradead.org