dmatest: gather test results in the linked list

The patch provides a storage for the test results in the linked list. The
gathered data could be used after test is done.

The new file 'results' represents gathered data of the in progress test. The
messages collected are printed to the kernel log as well.

Example of output:
	% cat /sys/kernel/debug/dmatest/results
	dma0chan0-copy0: #1: No errors with src_off=0x7bf dst_off=0x8ad len=0x3fea (0)

The message format is unified across the different types of errors. A number in
the parens represents additional information, e.g. error code, error counter,
or status.

Note that the buffer comparison is done in the old way, i.e. data is not
collected and just printed out.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>