LU-13717 sec: filename encryption - digest support

LU-13717 sec: filename encryption - digest support

A number of operations are allowed on encrypted files without the key:
- read file metadata (stat);
- list directories;
- remove files and directories.
In order to present valid names to users, cipher text names are base64
encoded if they are short. Otherwise we compute a digested form of the
cipher text, made of the FID (16 bytes) followed by the second-to-last
cipher block (16 bytes), and we base64 encode this digested form for
presentation to user.
These transformations are carried out in the specific overlay
functions, that now need to know the fid of the file.

As the digested form does not contain the whole cipher text name,
server side needs to proceed to an operation by FID for requests such
as lookup and getattr. It also relies on the content of the LinkEA to
verify the digested form as received from client side.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I45d10a426373c2cfe0b92a58c351da452d085d7d
Reviewed-on: https://review.whamcloud.com/43392
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>