Support for decrypted volumes

Wow! Such support! Many amaze decrypted!
...
Seriously, when the volume is in the `decrypted' state, there's no key
yet, but the metadata already are present so an NTFS reader can't mount
the volume.
Use read_decrypt_sectors() instead of read()

The problem with read() appears when the NTFS boot sector is encrypted.
Delegate this problem to read_decrypt_sectors() as it's already
implemented in it.