if_wm.c 1.410 sometimes hangs / sndq drops

[Frank Kardel <kardel%netbsd.org@localhost>]

With -current as of 20160526T144443Z and if_wm.c 1.410
a stuck interface is abserved on following hardware

wm1 at pci11 dev 0 function 0: Intel i82583V (rev. 0x00)
wm1: interrupting at ioapic0 pin 18
wm1: PCI-Express bus
wm1: 2048 words FLASH, version 1.10.0, Image Unique ID 0000ffff
wm1: Ethernet address bc:5f:f4:98:32:84
makphy0 at wm1 phy 1: Marvell 88E1149 Gigabit PHY, rev. 1

when rsyncing. The interface is on the sending side and
the sndq drops increase dramatically:

net.interfaces.wm1.rcvq.drops = 0
net.interfaces.wm1.sndq.len = 0
net.interfaces.wm1.sndq.maxlen = 256
net.interfaces.wm1.sndq.drops = 6077

ifconfig down/up recover the interface.

Any ideas?

Frank