Add new subcommand to inspect-internal to dump file extents

[kdave]

This has an overlap with filefrag or xfs_io -c fiemap, printing the exact results of the FIEMAP ioctl. For btrfs there are cases where this output is not reliable for shared extents, so we'd like to have something that can be used to tweak for specific cases and not rely on 3rd party tools.

There's a tool for that in Zygo's https://github.com/Zygo/bees https://github.com/Zygo/bees/blob/master/src/fiewalk.cc, it depends on the project files so it needs to be converted to what btrfs-progs have.

The exact output formatting is not that important at the moment, we need the extent information.