The device reports the maximum number of bytes to be written before
ringing the doorbell (zero means unlimited).

This patch queries the max batch size and reports it back to the userspace
library.

Link: https://lore.kernel.org/r/20200722140312.3651-2-galpress@amazon.comReviewed-by: NDaniel Kranzdorf <dkkranzd@amazon.com>
Reviewed-by: NFiras JahJah <firasj@amazon.com>
Signed-off-by: NGal Pressman <galpress@amazon.com>
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>