fi defrag: add option to set compression level

[michel-slm]

When setting or changing compression options, a user might want to defragment the existing filesystem to recompress existing files with the new settings.

However, btrfs fi defrag currently does not support it, or if it does, it is not documented. This should probably be added?

 
❯ rpm -q btrfs-progs                
btrfs-progs-5.7-5.fc33.x86_64

~ 
❯ btrfs filesystem defrag --help
usage: btrfs filesystem defragment [options] <file>|<dir> [<file>|<dir>...]

    Defragment a file or a directory

    -r                  defragment files recursively
    -c[zlib,lzo,zstd]   compress the file while defragmenting
    -f                  flush data to disk immediately after defragmenting
    -s start            defragment only from byte onward
    -l len              defragment only up to len bytes
    -t size             target extent size hint (default: 32M)
    -v                  deprecated, alias for global -v option
    
    Global options:
    -v|--verbose       increase output verbosity
    
    Warning: most Linux kernels will break up the ref-links of COW data
    (e.g., files copied with 'cp --reflink', snapshots) which may cause
    considerable increase of space usage. See btrfs-filesystem(8) for
    more information.

[kdave]

Duplicate of #184, let's continue there.