Allow configurable compression algorithm for btrfs-defrag

[yitzhaq]

From the btrfs-filesystem man page, defrag section:

-c[<algo>]
    compress file contents while defragmenting. Optional argument selects the compression algorithm, zlib (default), lzo or zstd. Currently it’s not possible to select no compression. See also section EXAMPLES.

I read this as meaning that, when defragging, compression will always be applied. Since there currently is no way in btrfsmaintenance to specify a value option or a corresponding value, zlib is used, which may not be desirable. It would be great if this was a configurable option in the defaults file, so you could select e.g. zstd.

[boniek83]

This is surprising. I wouldn't expect that defrag will change my compression settings. Default should be whatever is set in fstab. I was expecting this to be the case.