[Bug 220198] igb stats double counting in 12-CURRENT

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=220198

            Bug ID: 220198
           Summary: igb stats double counting in 12-CURRENT
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: woodsb02 at freebsd.org

My networking statistics appear to be double counting on two of my FreeBSD
12-CURRENT boxes running r319025.

I discovered the double counting by comparing the output of "systat -ifstat"
(which reports 988 KB/s) with the output of "iftop -B" (which reports 491
KB/s). I believe systat uses the kernel interface counters to infer bandwidth
usage, whereas iftop gathers statistics using pcap. I know that iftop is the
correct one, because my internet speed only gives me around ~500 KB/s
unfortunately.

# systat -ifstat -match igb0
...
      Interface           Traffic               Peak                Total
           igb0  in    988.008 KB/s        988.008 KB/s           36.295 GB
                 out    16.953 KB/s         16.953 KB/s          295.774 GB


# iftop -i igb0 -B
...
TX:    cum:   31.7KB   peak:   9.69KB    rates:   9.69KB  7.92KB  7.92KB
RX:           1.54MB            491KB              491KB   394KB   394KB


The two interfaces in these boxes are:

$ pciconf -lvv | grep igb1 -A4
igb1 at pci0:4:0:0:        class=0x020000 card=0x00008086 chip=0x15398086 rev=0x03
hdr=0x00
    vendor     = 'Intel Corporation'
    device     = 'I211 Gigabit Network Connection'
    class      = network
    subclass   = ethernet

$ pciconf -lvv | grep igb0 -A4
igb0 at pci0:8:0:0:        class=0x020000 card=0x15331849 chip=0x15338086 rev=0x03
hdr=0x00
    vendor     = 'Intel Corporation'
    device     = 'I210 Gigabit Network Connection'
    class      = network
    subclass   = ethernet

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