dmaengine: dmatest: wrap src & dst data into a struct

This change wraps the data for the source & destination buffers into a
`struct dmatest_data`. The rename patterns are:
 * src_cnt -> src->cnt
 * dst_cnt -> dst->cnt
 * src_off -> src->off
 * dst_off -> dst->off
 * thread->srcs -> src->aligned
 * thread->usrcs -> src->raw
 * thread->dsts -> dst->aligned
 * thread->udsts -> dst->raw

The intent is to make a function that moves duplicate parts of the code
into common alloc & free functions, which will unclutter the
`dmatest_func()` function.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>