LU-13460 lfs: make function print_failed_tgt() work correctly

LU-13460 lfs: make function print_failed_tgt() work correctly

param->fp_xxx_index is the number of the specified targets, not
the specific index, so it can't be passed into llapi_obd_statfs()
directly to get the statfs information. Instead, all the targets
listed in param->fp_xxx_indexes should be passed one by one.

Also, $mdt_idx in sanity.sh test_56rb should be initialized with
the correct mdt index.

Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Change-Id: Idffdddac1b5b249aa903b97912c767826f3b146c
Reviewed-on: https://review.whamcloud.com/38959
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>