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.

Signed-off-by: George Amanakis <gamanakis@gmail.com>

include/sys/ddt.h

@@ -179,7 +179,7 @@ typedef struct ddt_ops {
 	int (*ddt_op_count)(objset_t *os, uint64_t object, uint64_t *count);
 } ddt_ops_t;
 
-#define	DDT_NAMELEN	80
+#define	DDT_NAMELEN	102
 
 extern void ddt_object_name(ddt_t *ddt, enum ddt_type type,
     enum ddt_class clazz, char *name);