LU-4397 utils: fix lfs_df loop for disconnected client

LU-4397 utils: fix lfs_df loop for disconnected client

If the client is disconnected from the MDT on which "lfs df" is
run (typically MDT0 for the mountpoint), then the file open can
fail with -ESHUTDOWN, causing mntdf() to loop forever trying to
find other MDTs or OSTs to get statfs data from.

The caller of llapi_obd_statfs() cannot tell the difference between
an error from open() and ioctl(IOC_OBD_STATFS), so llapi_obd_statfs()
should return a fatal error if the open failed since all of the
later opens will fail as well.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I05d48d5b6680c382c9e05c02fd82a9a200500c1e
Reviewed-on: http://review.whamcloud.com/8949
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>