iscsi-target: use '%*ph' specifier to dump hex buffer

Instead of pushing each byte via stack the %*ph specifier allows to supply just
a pointer and length of the buffer. The patch converts code to use the
specifier.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>