lib/decompress: use designated initializers for struct compress_format

Switch 'compressed_formats[]' to the more modern and flexible designated
initializers.  This improves readability and allows struct fields to be
reordered.  Also use a more concise sentinel marker.

Remove the curly braces around the for loop while we're at it.

No functional changes intended.

Link: https://lkml.kernel.org/r/20250910232350.1308206-2-thorsten.blum@linux.dev


Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: David Sterba <dsterba@suse.com>
Cc: Nick Terrell <terrelln@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>