LU-12275 sec: support truncate for encrypted files

Truncation of encrypted files is not a trivial operation. The page
corresponding to the point where truncation occurs must be read,
decrypted, zeroed after truncation point, re-encrypted and then
written back.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I834f9372913d7051b1e0821515d3fea0873ffd78
Reviewed-on: https://review.whamcloud.com/37794
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>