EncryptInplace: fsync cryptofd before reporting success

fsync() the cryptofd when done writing to it.  Without this, any
remaining dirty pages in the crypto_blkdev's page cache (which there
might be a lot of, even as much as all the data that was written) won't
be flushed to disk until the cryptofd is closed, which ignores I/O
errors and is also after we already reported 100% completion.

There wasn't an fsync() in the original version either, so we've been
getting by without it, but it seems it should be there.

Change-Id: Idd1be3ae67ce96ecf3946b9efb9fc57414f5805a