block: nr_phys_segments needs to be zero for REQ_OP_WRITE_ZEROES

Fix a regression introduced when removing bi_phys_segments for Write Zeroes
requests, which need to have a segment count of zero, as they don't have a
payload.

Fixes: 14ccb66b ("block: remove the bi_phys_segments field in struct bio")
Reported-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>