RDMA/hfi1: Stop using seq_get_buf in _driver_stats_seq_show

Just use seq_write to copy the stats into the seq_file buffer instead of
poking holes into the seq_file abstraction.

Link: https://lore.kernel.org/r/20210810151711.1795374-1-hch@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@cornelisnetworks.com>
Tested-by: Mike Marciniszyn <mike.marciniszyn@cornelisnetworks.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>