dm ioctl: Fix compilation warning

In retrieve_status(), when copying the target type name in the
target_type string field of struct dm_target_spec, copy at most
DM_MAX_TYPE_NAME - 1 character to avoid the compilation warning:

warning: ‘__builtin_strncpy’ specified bound 16 equals destination
size [-Wstringop-truncation]

when compiling with W-1.
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-ioctl.c

@@ -1168,7 +1168,7 @@ static void retrieve_status(struct dm_table *table,
 		spec->sector_start = ti->begin;
 		spec->length = ti->len;
 		strncpy(spec->target_type, ti->type->name,
-			sizeof(spec->target_type));
+			sizeof(spec->target_type) - 1);
 
 		outptr += sizeof(struct dm_target_spec);
 		remaining = len - (outptr - outbuf);