In host control mode the host is the originator of map requests. To not
flood the device with map requests, use a simple throttling mechanism that
limits the number of in-flight map requests.

Link: https://lore.kernel.org/r/20210712095039.8093-10-avri.altman@wdc.comReviewed-by: Daejun Park <daejun7.park@samsung.com>
Signed-off-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>