ifpps: fix bug_on trigger when parsing irq number

When parsing irq numbers on e.g. 24 CPU machines, procfs lines can get
very long, so our static temporary buffer size will fail here with:

  Assertion `!(stats->irq_nr == 0)' failed.

So allocate it relative to the CPU number.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>