handle large discard/write_zeroes without buffer panic#18
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Metadata-only ublk ops (DISCARD, WRITE_ZEROES, FLUSH) don't use the data
buffer, but dispatch_io was slicing the buffer to the request length before
checking the op type. Since max_discard_sectors allows 16MB requests against
a 512KB per-tag buffer, mkfs.ext4 discards panic on the slice bounds check.
Move the dispatch_passthrough call before the buffer slice, matching what
the zero-copy path already does.