DF-3279 baseline measurement (stock kernel #0).

    $ ifconfig tap0 create; ifconfig bridge0 create
    $ vmstat -m | grep -E "^ *devbuf"
                 devbuf   24.6K   3.12M       0    390M    49.1K
    $ ./span_leak 20000
    [+] running 20000 spanport add/del cycles on bridge0/tap0
    [+] done: 20000 cycles
    $ vmstat -m | grep -E "^ *devbuf"
                 devbuf   44.2K   5.40M       0    390M    88.2K

Delta: +2.28 MB / 20000 cycles = ~114 bytes leaked per cycle.

Earlier cross-check with 20000 cycles against total memory:
In-Use 38.8M -> 41.1M (+2.3M), consistent.

Fixed kernel (#1, bif_info freed in bridge_delete_span):
                 devbuf     236    266K       0    390M      347   (before)
                 devbuf     236    266K       0    390M    39.4K   (after 20000 cycles)
    -> zero growth.