LU-3864 lov: return minimal FIEMAP for released files

LU-3864 lov: return minimal FIEMAP for released files

Since st_blocks = NULL is returned for released files, FIEMAP
should at least return a minimal mapping to make users aware
that file contains data but it is not immediately available.
This will make coreutils and tools such tar happy and have
them presume file is sparse.

Also, add a new test_228 in sanity-hsm to verify it works
for "[cp,tar] --sparse" commands.

Also fix a LBUG ("lov_fiemap()) ASSERTION( fm_local ) failed")
likely to occur when no-object/ENOMEM conditions and also now
when released.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: Idc59f755d407fd34e7095898c68b4aceb5bf9c28
Reviewed-on: http://review.whamcloud.com/7584
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Aurelien Degremont <aurelien.degremont@cea.fr>