Based on user feedback and actual problems with compression property,
there's no support to unset any compression options, or to force no
compression flag.

Note: This has changed recently in e2fsprogs 1.46.2, 'chattr +m'
(setting NOCOMPRESS).

In btrfs properties, the empty value should really mean reset to
defaults, for all properties in general. Right now there's only the
compression one, so this change should not cause too many problems.

Old behaviour:

  $ lsattr file
  ---------------------- file
  # the NOCOMPRESS bit is set
  $ btrfs prop set file compression ''
  $ lsattr file
  ---------------------m file

This is equivalent to 'btrfs prop set file compression no' in current
btrfs-progs as the 'no' or 'none' values are translated to an empty
string.

This is where the new behaviour is different: empty string drops the
compression flag (-c) and nocompress (-m):

  $ lsattr file
  ---------------------- file
  # No change
  $ btrfs prop set file compression ''
  $ lsattr file
  ---------------------- file
  $ btrfs prop set file compression lzo
  $ lsattr file
  --------c------------- file
  $ btrfs prop get file compression
  compression=lzo
  $ btrfs prop set file compression ''
  # Reset to the initial state
  $ lsattr file
  ---------------------- file
  # Set NOCOMPRESS bit
  $ btrfs prop set file compression no
  $ lsattr file
  ---------------------m file

This obviously brings problems with backward compatibility, so this
patch should not be backported without making sure the updated
btrfs-progs are also used and that scripts have been updated to use the
new semantics.

Summary:

- old kernel:
  no, none, "" - set NOCOMPRESS bit
- new kernel:
  no, none - set NOCOMPRESS bit
  "" - drop all compression flags, ie. COMPRESS and NOCOMPRESS
Signed-off-by: David Sterba <dsterba@suse.com>