ddt: reduce DDT_NAMELEN

This is the buffer size passed to ddt_object_name(), to expand the
DMU_POOL_DDT format. That format inserts the table checksum, class and
type names, which as I write this are max 6, 9 and 3, respectively.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #15908