Another issue with ixl(4) with netmap

[Garrett Wollman]

I noticed that a large number -- but by no means all -- of the packets
captured using libpcap on a netmap'ified ixl(4) interface show up as
truncated -- usually by exactly four bytes.  They show up in tcpdump
like this:

18:10:05.348735 IP truncated-ip - 4 bytes missing! 128.30.xxx.xxx.443 > yyy.yyy.yyy.yyy.54711: Flags [P.], seq 3151716053:3151716112, ack 3109956633, win 31, length 59
18:10:05.348735 IP truncated-ip - 4 bytes missing! zzz.zzz.zzz.zzz.17376 > 128.30.aaa.aaa.80: Flags [.], ack 1771224170, win 12775, options [nop,nop,sack 1 [|tcp]>
18:10:05.348735 IP truncated-ip - 4 bytes missing! zzz.zzz.zzz.zzz.17376 > 128.30.aaa.aaa.80: Flags [.], ack 1, win 12775, options [nop,nop,sack 1 [|tcp]>

This does not occur when the same interface is used with BPF (but
libpcap can't keep up with the traffic when using BPF).  Has anyone
see this before, and if so, what was the solution?  (This is on 10.2
with the ixl(4) updates merged from 10-stable.)

-GAWollman