net/sunrpc/cache: simplify code by using hex_pack_byte()

hex_pack_byte() is a fast way to convert a byte in its ASCII representation. We
may use it instead of custom approach.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>