LU-4364 fld: add error handler in fldb_seq_start

LU-4364 fld: add error handler in fldb_seq_start

Check return value for load/key in fldb_seq_start,
in case there are no entries in FLDB.

Only insert special entries to the local FLDB in MDT0,
since they are all in MDT0.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: If1281800ad75396c96bd47021742ea21da7abad8
Reviewed-on: http://review.whamcloud.com/8534
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Tested-by: Oleg Drokin <oleg.drokin@intel.com>