btrfs-progs: Introduce function to fix super block total bytes

btrfs-progs: Introduce function to fix super block total bytes

Recent kernel (starting from v4.6) will refuse to mount if super block
total bytes is smaller than all devices' size.

This makes end user unable to do anything to their otherwise quite
healthy fs.

To fix such problem, introduce repair function to fix it on an unmounted
filesystem.

Reported-by: Asif Youssuff <yoasif@gmail.com>
Reported-by: Rich Rauenzahn <rrauenza@gmail.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>