Avoid allocations during runtime

[JJL772]

Description

Pre-allocates scratch buffers so we don't have to keep calling kmalloc during dmaReadIndex and such.

Introduces the following limitations:

• Max number of indices that can be returned at once is cfgRxCount + cfgTxCount
• Max number of buffers that can be read or written at once is cfgRxCount + cfgTxCount

If those conditions are violated, the ioctl returns with error.

I am not sure if the DAQ is trying to read/write/return more than its allotted amount.