dm integrity: Use %*ph for printing hexdump of a small buffer

The kernel already has a helper to print a hexdump of a small
buffer via pointer extension. Use that instead of open coded
variant.

In long term it helps to kill pr_cont() or at least narrow down
its use.

Note, the format is slightly changed, i.e. the trailing space is
always printed. Also the IV dump is limited by 64 bytes which seems
fine.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>