raid5: remove a call to get_start_sect

The block layer always remaps partitions before calling into the
->make_request methods of drivers.  Thus the call to get_start_sect in
in_chunk_boundary will always return 0 and can be removed.

Reviewed-by: Shaohua Li <shli@fb.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>