[PATCH v3 00/13] add compression ioctl support

[David Disseldorp]

This patch series adds support for the FSCTL_GET_COMPRESSION and
FSCTL_SET_COMPRESSION ioctls, as well as the reporting of the current
compression state via the FILE_ATTRIBUTE_COMPRESSED flag.

Hooks are added to the Btrfs VFS module, which translates such requests
into corresponding FS_IOC_GETFLAGS and FS_IOC_SETFLAGS ioctls.

Changes since v2 (thanks for the feedback Jeremy):
- Check for valid fsp file descriptors
- Rebase atop filesystem specific selftest changes
- Change compression fsctl permission checks to match Windows behaviour
  + Add corresponding smbtorture test

Changes since v1:
- Only use smb_fname and fsp args with GET_COMPRESSION. The smb_fname
  argument is only needed for the dosmode() code-path, fsp is used
  everywhere else.
- Add an extra SetInfo(FILE_ATTRIBUTE_COMPRESSED) test.