LU-8731 utils: propagate errors in lfs df

LU-8731 utils: propagate errors in lfs df

Add llapi_obd_fstatfs() which does the same thing as
llapi_obd_statfs() but takes an open file descriptor instead of a
path. Refector the handler for 'lfs df' to use llapi_obd_fstatfs(),
thereby avoiding opening the mount point for each target and making
the error conditions easier to understand. Propagate errors from
llapi_obd_fstatfs() as the exit status of 'lfs df'.

In conf-sanity.sh test_64() allow 'lfs df' to fail when a target is
offline.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Iabfc92a65571b1a277de7fd42431f5b7e45ad440
Reviewed-on: http://review.whamcloud.com/23286
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Hongchao Zhang <hongchao.zhang@intel.com>