stable/9 igb(4) panic, soabort

[Sean Bruno]

Since I saw other panics on our stable/9 I started poking around and found this one lying around too.
And, I have a crashdump here as well.  Not sure about reproduction, but I see it happened on two seperate servers
over the course of the day.  Here is one of them.

<boot snippet>
igb0: <Intel(R) PRO/1000 Network Connection version - 2.3.4> port 0xe880-0xe89f mem 0xfbe60000-0xfbe7ffff,0xfbe40000-0xfbe5ffff,0xfbeb8000-0xfbebbfff irq 32 at device 0.0 on pci5
igb0: Using MSIX interrupts with 5 vectors
igb0: Ethernet address: 44:1e:a1:61:0a:76
igb0: Bound queue 0 to cpu 0
igb0: Bound queue 1 to cpu 1
igb0: Bound queue 2 to cpu 2
igb0: Bound queue 3 to cpu 3
igb1: <Intel(R) PRO/1000 Network Connection version - 2.3.4> port 0xec00-0xec1f mem 0xfbee0000-0xfbefffff,0xfbec0000-0xfbedffff,0xfbebc000-0xfbebffff irq 42 at device 0.1 on pci5
igb1: Using MSIX interrupts with 5 vectors
igb1: Ethernet address: 44:1e:a1:61:0a:77
igb1: Bound queue 0 to cpu 4
igb1: Bound queue 1 to cpu 5
igb1: Bound queue 2 to cpu 6
igb1: Bound queue 3 to cpu 7


<panic>
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Unread portion of the kernel message buffer:
panic: soabort: so_count
cpuid = 6
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2a
kdb_backtrace() at kdb_backtrace+0x37
panic() at panic+0x1d8
soabort() at soabort+0x99
syncache_expand() at syncache_expand+0x30c
tcp_input() at tcp_input+0xe5c
ip_input() at ip_input+0xbd
netisr_dispatch_src() at netisr_dispatch_src+0x152
ether_demux() at ether_demux+0x17d
ether_nh_input() at ether_nh_input+0x1fe
netisr_dispatch_src() at netisr_dispatch_src+0x152
igb_rxeof() at igb_rxeof+0x3a4
igb_msix_que() at igb_msix_que+0xd5
intr_event_execute_handlers() at intr_event_execute_handlers+0x6a
ithread_loop() at ithread_loop+0xb4
fork_exit() at fork_exit+0x135
fork_trampoline() at fork_trampoline+0xe
--- trap 0, rip = 0, rsp = 0xffffff846c888cf0, rbp = 0 ---
Uptime: 1h17m18s
Dumping 1141 out of 16353
MB:..2%..12%..22%..31%..41%..51%..61%..71%..82%..92%