staging/lustre/fld: refer to MDT0 for fld lookup in some cases

It is possible that when fld client is trying to lookup seq
on one of MDT, but the connection between the client and the MDT
is not being initialized yet, especially during striped dir creation,
because client will only send create req to the master MDT, then
master MDT will distribute the operation to all of other MDT, instead
of client distributing these requests, which will usually trigger
the connection.

In this case, we will send the fld request to MDT0, since it has
all of location information.

Signed-off-by: wang di <di.wang@intel.com>
Reviewed-on: http://review.whamcloud.com/11780
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4855


Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>