btrfs: print-tree: use string format for key names

btrfs: print-tree: use string format for key names

There's a warning when -Wformat=2 is used:

fs/btrfs/print-tree.c: In function ‘key_type_string’:
fs/btrfs/print-tree.c:424:17: warning: format not a string literal and no format arguments [-Wformat-nonliteral]
  424 |                 scnprintf(buf, buf_size, key_to_str[key->type]);

We're printing fixed strings from a table so there's no problem but
let's fix the warning so we could enable the warning in fs/btrfs/.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>