dm ioctl: replace deprecated strncpy with strscpy_pad

`strncpy` is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

We expect `spec->target_type` to be NUL-terminated based on its use with
a format string after `dm_table_add_target()` is called
| 	r = dm_table_add_target(table, spec->target_type,
| 				(sector_t) spec->sector_start,
| 				(sector_t) spec->length,
| 				target_params);
... wherein `spec->target_type` is passed as parameter `type` and later
printed with DMERR:
|       DMERR("%s: %s: unknown target type", dm_device_name(t->md), type);

It appears that `spec` is not zero-allocated and thus NUL-padding may be
required in this ioctl context.

Considering the above, a suitable replacement is `strscpy_pad` due to
the fact that it guarantees NUL-termination whilst maintaining the
NUL-padding behavior that strncpy provides.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90


Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>