LU-15176 sec: allow subdir mount of encrypted dir

LU-15176 sec: allow subdir mount of encrypted dir

In case of sub-directory mount of an encrypted directory, we need to
retrieve the encryption context of the root inode of the filesystem.
This is done by making the MDT return this upon getattr reply.

Also add sanity-sec test_60 to exercise this capability.

Lustre-change: https://review.whamcloud.com/45407
Lustre-commit: faf057b46bc770a1a69cacd59e65a40a4b18b9fd

Fixes: 40d91eafe2 ("LU-12275 sec: atomicity of encryption context getting/setting")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ic7a273813533f2904225011b247cdfe995ce9be8
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/45909
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>