samples/bpf: Fix summary per-sec stats in xdp_sample_user

samples/bpf: Fix summary per-sec stats in xdp_sample_user

sample_summary_print() uses accumulated period to calculate and display
per-sec averages. This period gets incremented by sampling interval each
time a new sample is formed, and thus equals to the number of samples
collected multiplied by this interval.

However, the totals are being calculated differently, they receive current
sample statistics already divided by the interval gotten as a difference
between sample timestamps for better precision -- in other words, they are
being incremented by the per-sec values each sample.

This leads to the excessive division of summary per-secs when interval != 1
sec. It is obvious pps couldn't become two times lower just from picking a
different sampling interval value:

  $ samples/bpf/xdp_redirect_cpu -p xdp_prognum_n1_inverse_qnum -c all
    -s -d 6 -i 1
  < snip >
    Packets received    : 2,197,230,321
    Average packets/s   : 22,887,816
    Packets redirected  : 2,197,230,472
    Average redir/s     : 22,887,817
  $ samples/bpf/xdp_redirect_cpu -p xdp_prognum_n1_inverse_qnum -c all
    -s -d 6 -i 2
  < snip >
    Packets received    : 159,566,498
    Average packets/s   : 11,397,607
    Packets redirected  : 159,566,995
    Average redir/s     : 11,397,642

This can be easily fixed by treating the divisor not as a period, but rather
as a total number of samples, and thus incrementing it by 1 instead of
interval. As a nice side effect, we can now remove so-named argument from a
couple of functions. Let us also create an "alias" for sample_output::rx_cnt::pps
named 'num' using a union since this field is used to store this number (period
previously) as well, and the resulting counter-intuitive code might've been a
reason for this bug.

Fixes: 156f886cf697 ("samples: bpf: Add basic infrastructure for XDP samples")
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Reviewed-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/bpf/20211111215703.690-1-alexandr.lobakin@intel.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>