Sorry, i didn't realize how easy it is to pull dmesg from core files:

IPv4 ESP input: no key association found for spi 9866193
uvm_fault(0xc0a30de0, 0xc2912000, 1) -> 0xe
fatal page fault in supervisor mode
trap type 6 code 0 eip c0161a7d cs 8 eflags 10282 cr2 c2912014 ilevel 5
panic: trap
Begin traceback...
uvm_fault(0xcb7e87e0, 0, 1) -> 0xe
fatal page fault in supervisor mode
trap type 6 code 0 eip c05062a0 cs 8 eflags 10246 cr2 6 ilevel 5
panic: trap
Faulted in mid-traceback; aborting...
dumping to dev 0,1 offset 543351
dump 1014 1013 1012 1011 1010 1009 1008 1007 1006 1005 1004 1003 1002 1001 1000
[....] 4 3 2 1

I don't think the 'no key association found' is relevant; it's not
consistently present.  The rest of them have the same fault address.

(gdb) x/i 0xc0161a7d
0xc0161a7d <fr_makefrip+349>:   cmp    0xc(%edx),%eax

This maps back to line 773 of fil.c.  I will install a kernel with the
not-yet-pulled-up fix for sparc64 alignment and see how that goes.  I
don't understand why the change would make any difference on i386,
though.