loop: allow user to set the queue depth

loop: allow user to set the queue depth

Instead of hardcoding queue depth allow user to set the hw queue depth
using module parameter. Set default value to 128 to retain the existing
behavior.

Signed-off-by: Chaitanya Kulkarni <[email protected]>
Reviewed-by: Himanshu Madhani <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>