Fix gcc10.1 truncation error

Fix gcc10.1 truncation error

gcc10.1 complains with:

../../include/sys/dmu.h:373:24: error: ‘%s’ directive output may be
truncated writing up to 95 bytes into a region of size 75
[-Werror=format-truncation=]
  373 | #define DMU_POOL_DDT   "DDT-%s-%s-%s"
      |                        ^~~~~~~~~~~~~~
../../module/zfs/ddt.c:256:37: note: in expansion of macro
‘DMU_POOL_DDT’
  256 |  (void) snprintf(name, DDT_NAMELEN, DMU_POOL_DDT,
      |                                     ^~~~~~~~~~~~
../../include/sys/dmu.h:373:32: note: format string is defined here
  373 | #define DMU_POOL_DDT   "DDT-%s-%s-%s"
      |                                ^~
../../module/zfs/ddt.c:256:9: note: ‘snprintf’ output 7 or more bytes
(assuming 102) into a destination of size 80
  256 |  (void) snprintf(name, DDT_NAMELEN, DMU_POOL_DDT,
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  257 |      zio_checksum_table[ddt->ddt_checksum].ci_name,
      |      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  258 |      ddt_ops[type]->ddt_op_name, ddt_class_name[class]);
      |      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Increasing DTT_NAMELEN fixes it.

Reviewed-By: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #10433