LU-12275 sec: decryption for read path

LU-12275 sec: decryption for read path

With the support for encryption, all files need to be opened with
fscrypt_file_open(). fscrypt will retrieve encryption context if
file is encrypted, or immediately return if not.
Decryption itself is carried out in osc_brw_fini_request(), right
after the reply has been received from the server.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I8f8f87eb8e07e35e1a4e6cc157ceddfef6934753
Reviewed-on: https://review.whamcloud.com/36145
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <farr0186@gmail.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>