blk-wbt: account any writing command as a write

We currently special case WRITE and FLUSH, but we should really
just include any command with the write bit set. This ensures
that we account DISCARD.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>