shared/dm-util: use strncpy_exact() to silence gcc

shared/dm-util: use strncpy_exact() to silence gcc

With gcc-9.1.1-2.fc31.x86_64 and -Doptimization=2:
../src/shared/dm-util.c: In function ‘dm_deferred_remove’:
../src/shared/dm-util.c:35:9: warning: ‘strncpy’ specified bound 128 equals destination size [-Wstringop-truncation]
   35 |         strncpy(dm.name, name, sizeof(dm.name));
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

gcc is plain wrong here, because we checked strlen(name) a few lines above, so
there can be no truncation and even the terminator always fits. But let's avoid
the warning.