kern/49219: if_arp holds just one pending packet and drops all older packets

[ozaki-r%netbsd.org@localhost]

>Number:         49219
>Category:       kern
>Synopsis:       if_arp holds just one pending packet and drops all older 
>packets
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    kern-bug-people
>State:          open
>Class:          change-request
>Submitter-Id:   net
>Arrival-Date:   Fri Sep 19 10:10:00 +0000 2014
>Originator:     Ryota Ozaki
>Release:        current
>Organization:
>Environment:
any
>Description:
When ARP is required on sending a packet, if_arp holds the packet
until a destination address of the packet is resolved. Currently just
one packet is held by an interface and it will be dropped if a new
packet that heads to the same address is being sent via the interface.

In many cases, this limitation doesn't matter. However, sometimes
it annoys us. For example, my test script for bridge randomly failed
due to this behavior :-/ For another example, if a first outgoing
packet is fragmented, its first fragment will be always dropped.
>How-To-Repeat:

>Fix:
Hold pending packets with a list on an interface as same as FreeBSD and OpenBSD.