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.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I2a0ee360f724da1df49b2be0df986d52e06f45fd
Reviewed-on: https://review.whamcloud.com/46167
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>