LU-14472 quota: skip non-exist or inact tgt for lfs_quota

LU-14472 quota: skip non-exist or inact tgt for lfs_quota

The nonexistent or inactive targets (MDC or OSC) should be skipped
for "lfs quota".

Lustre-change: https://review.whamcloud.com/41771
Lustre-commit: b54b7ce43929ce7ff6e48cd219623c264ca6b6b3

Change-Id: I25eece413715e4e05dd94ccbfd101220da7477f9
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/48171
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>