LU-11376 lov: new foreign LOV format

LU-11376 lov: new foreign LOV format

This patch introduces a new layout/LOV format in order to
allow to specify an arbitrary external reference for a file
in Lustre namespace.
The new LOV format is made of {newmagic, length, type, flags,
string[length]} to be as flexible as possible.
Foreign file can be created by using the open(O_LOV_DELAY_CREATE) +
ioctl(LL_IOC_LOV_SETSTRIPE) operations and it can only be and remain
an empty file until removed.
A new API method llapi_file_create_foreign() has been introduced
and "lfs [[get,set]stripe,find" modified to understand new layout.
The idea behind this is to provide Lustre namespace support and
layout prefetch/caching under layout protection, for user/external
usage.

Code has been added for lfsck to handle foreign files, and
a new sub-test has been added in sanity-lfsck in order to verify
if does not break foreign file and that reverse is also true.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I5d9c0642fe8e7009c30918bfa946cac7c00c9af8
Reviewed-on: https://review.whamcloud.com/33755
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>