igb(4) panic: already enqueue

[Konstantin Belousov]

On a relatively old test machine, dual-socket X7DWU supermicro, Xeon
X5272, with on-board 82575EB igb controllers, running the stress2
udp blast test immediately causes the following

panic: buf=0xfffff8001aa5c800 already enqueue at 1445 prod=1315 cons=1445
cpuid = 3
KDB: stack backtrace:
db_trace_self_wrapper() at 0xffffffff802839ab = db_trace_self_wrapper+0x2b/frame 0xfffffe0123570600
vpanic() at 0xffffffff8033bd96 = vpanic+0x126/frame 0xfffffe0123570640
panic() at 0xffffffff8033be23 = panic+0x43/frame 0xfffffe01235706a0
igb_mq_start() at 0xffffffff80bcb228 = igb_mq_start+0xb8/frame 0xfffffe0123570710   
ether_output() at 0xffffffff804072a8 = ether_output+0x5c8/frame 0xfffffe0123570780  
ip_output() at 0xffffffff80430be1 = ip_output+0xe91/frame 0xfffffe0123570860
udp_send() at 0xffffffff804a5fc4 = udp_send+0x834/frame 0xfffffe0123570910
sosend_dgram() at 0xffffffff803b517b = sosend_dgram+0x36b/frame 0xfffffe0123570990  
soo_write() at 0xffffffff8039dbb9 = soo_write+0x49/frame 0xfffffe01235709d0
dofilewrite() at 0xffffffff803956c5 = dofilewrite+0x85/frame 0xfffffe0123570a20
kern_writev() at 0xffffffff803953d5 = kern_writev+0x65/frame 0xfffffe0123570a70
sys_write() at 0xffffffff80395363 = sys_write+0x63/frame 0xfffffe0123570ac0
amd64_syscall() at 0xffffffff8053ae7d = amd64_syscall+0x28d/frame 0xfffffe0123570bf0

Test is available at
svn:base/user/pho/stress2/testcases/udp, invocation line is
BLASTHOST=192.168.5.1 ./udp -t 10m -i 50 -h -v
Substitute the BLASTHOST with the ip address of a scratch box.

This is on the yesterday HEAD.  Could you, please, fix this ?
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