LU-11376 lmv: new foreign LMV format

LU-11376 lmv: new foreign LMV format

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

This patch is the LMV/dirs complement of LOV/files previous change
(Change-Id: I5d9c0642fe8e7009c30918bfa946cac7c00c9af8) and has
been rebased on top of the latter along with some with obvious
mutualizations and simplifications.

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

Also fixes a bug causing SEGVs during
"lfs find [--mdt-count=[+,-]<count>, --mdt-hash=<hashtype>]" when
handling a file (ie, "DIR *dir" is NULLL) in cb_find_init().

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I3721b8f14578bf926a92da76375dae92dc8d764d
Reviewed-on: https://review.whamcloud.com/34087
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>