lz4: add setting of comp_opts structure

Currently the comp_opts structure has a version field.

The code expects a comp_opts structure to be present, and
it expects a version of LZ4_LEGACY.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
lz4: add flags field to comp_opts structure

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
mksquashfs: add additional comment concerning use of compressor_extract_options

Add additional comment to the effect that compressor_extract_options is also
used to ensure that we know how decompress a filesystem compressed with the
compression options in the filesystem.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
unsquashfs: Call compressor_extract_options

Unsquashfs should have been calling compressor_extract_options to
check that it knew how to decompress a filesystem compressed with
the options stored in the filesystem.

As it happens xz compression up till now has been the only
compressor to use compression options, and the code has always
been able to decompress xz compressed filesystems whatever the
compression options used, and so the check hasn't been needed.  But this
may change with other compressors.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>