CVE-2026-64509: rust: block: fix GenDisk cleanup paths

In the Linux kernel, the following vulnerability has been resolved:

rust: block: fix GenDisk cleanup paths

GenDiskBuilder::build() still has fallible work after blkmqallocdisk(), but its error path only recovers the foreign queue data. That leaks the temporary gendisk and requestqueue until later teardown. If the caller moved the last Arc<TagSet<T>> into build(), the leaked queue can retain blk-mq state after the tag set is dropped.

Fix the pre-registration failure path by dropping the temporary gendisk reference with putdisk() before recovering queuedata, so diskrelease() can tear down the owned queue.

Also pair GenDisk::drop() with putdisk() after delgendisk(). Once a Rust GenDisk has been added with deviceadddisk(), delgendisk() only unregisters it; the final gendisk reference still has to be dropped to complete the release path.