md: raid6: clean up the style of raid6test/test.c

Clean up the coding style in raid6test/test.c.  Break it apart into
subfunctions to make the code more readable.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>