avoid several strncpy-induced buffer overruns

avoid several strncpy-induced buffer overruns

* restore.c (main): Ensure strncpy-copied dir_name is NUL-terminated.
* btrfsctl.c (main): Likewise, for a command-line argument.
* utils.c (multiple functions): Likewise.
* btrfs-list.c (add_root): Likewise.
* btrfslabel.c (change_label_unmounted): Likewise.
* cmds-device.c (cmd_add_dev, cmd_rm_dev, cmd_scan_dev): Likewise.
* cmds-filesystem.c (cmd_resize): Likewise.
* cmds-subvolume.c (cmd_subvol_create, cmd_subvol_delete, cmd_snapshot):
Likewise.

Reviewed-by: Josef Bacik <josef@redhat.com>