DF-2902 fixed-kernel validation (after fix.diff, kernel #1)
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LEG A (blksize=0x40000):
mbrprobe: dev=vn0 blksize=0x40000 blocks=64 media_size=16777216
mbrprobe: mbrinit returned 5 nslices=2     (5 = EIO — rejected, no read issued)
mbrprobe: done (no crash)

LEG B (blksize=0x80000000):
mbrprobe: dev=vn0 blksize=0x80000000 blocks=64 media_size=137438953472
mbrprobe: mbrinit returned 5 nslices=2     (5 = EIO — rejected, no read issued)
mbrprobe: done (no crash)

uptime after both legs: up 2 mins, load averages: 0.06, 0.03, 0.01

=> Both baseline panics GONE; hostile block sizes are rejected with EIO
   before any I/O is issued.