dissect-image: open dissected or decrypted partitions and mount through the file...

dissect-image: open dissected or decrypted partitions and mount through the file descriptor

If multiple services with the same encrypted image are simultaneously
starting, one may deactivate the dm device while others using it.
Or, similary, after (regular) partitions are dissected, another process
may try to remove them before we mount them.

To prevent such situations, let's keep the dissected and decrypted
partitions opened. Then, use the file descriptors when we mount the
partitions.

Fixes #24617.