LU-12275 sec: O_DIRECT for encrypted file

LU-12275 sec: O_DIRECT for encrypted file

Add O_DIRECT support for encrypted files.
By default, fscrypt does not support O_DIRECT because it needs
pagecache pages to proceed.
With Lustre, we can make use of pages being used for sending RPCs.
They can be twisted so that they have a proper mapping and index,
suitable for encryption/decryption.

One of the benefits of O_DIRECT support for encrypted files is that
we get support for mirroring at the same time.

Test-Parameters: testlist=sanity-sec envdefinitions=ONLY="36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 52" clientdistro=el8.1 fstype=ldiskfs mdscount=2 mdtcount=4
Test-Parameters: testlist=sanity-sec envdefinitions=ONLY="36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 52" clientdistro=el8.1 fstype=zfs mdscount=2 mdtcount=4
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I12f61c44b55f3a454f38016200f81eb735ab8f18
Reviewed-on: https://review.whamcloud.com/38967
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>