vfs: add [GET/SET]_COMPRESSION hooks

vfs: add [GET/SET]_COMPRESSION hooks

The VFS interfaces are sychronous, as the operations only modify
meta-data.
These hooks are dependent on support for transparent compression by the
underlying filesystem - vfs_default returns INVALID_DEVICE_REQUEST.
Support for other filesystems providing transparent comression, such as
Btrfs and ZFS, can be added in future.

The get_compression function takes fsp and smb_fname arguments. The
smb_fname argument is needed due to the current dosmode() code-path.

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>