raid6test: use prandom_bytes()

Use prandom_bytes() to generate random bytes for test data.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Dan Williams <djbw@fb.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>