LU-15176 sec: present .fscrypt in subdir mount

LU-15176 sec: present .fscrypt in subdir mount

fscrypt userspace tool works with a .fscrypt directory at the root of
the file system. In case of subdirectory mount, we virtually present
this .fscrypt directory at the root of the mount point so that fscrypt
can be used. This makes it possible to even do a subdirectory mount of
an encrypted directory, making clients access encrypted content only.
Internally, the .fscrypt directory is always stored at the root of
Lustre.

Lustre-change: https://review.whamcloud.com/46167
Lustre-commit: c12378fba7f0aa2f25de2b295ee110e2932f2a39

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I2a0ee360f724da1df49b2be0df986d52e06f45fd
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46888
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>