mkfs.bfs: fix memory leaks and weak code

mkfs.bfs: fix memory leaks and weak code

- use size_t to store strlen() result
- init superblock with the default volume and fsname
- don't use strdup(), it's unnecessary as getopt_long() does not
  modify arguments
- don't use memcpy() as we need to check string sizes
- restrict verbose output 6 bytes

Addresses: https://github.com/util-linux/util-linux/pull/3488
Signed-off-by: Karel Zak <kzak@redhat.com>