LU-14677 sec: migrate/extend/split on encrypted file

LU-14677 sec: migrate/extend/split on encrypted file

lfs migrate/extend/split makes use of volatile files to swap layouts.
When operation is carried out on an encrypted file, the volatile file
must be assigned the same encryption context as the original file, so
that data moved/copied to different OSTs is identical to the original
file's.
Also update sanity-sec test_52 to exercise these commands.

Lustre-change: https://review.whamcloud.com/43878
Lustre-commit: 09c558d16f0a80f436522edde89367c088fe2055

Change-Id: I3878b5e9e6d3738dfee0ce0f89a3646e6a7b976f
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-on: https://review.whamcloud.com/43879
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>