test_hexdump: switch to memcmp()

Better to use memcmp() against entire buffer to check that nothing is
happened to the data in the tail.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>