NFSv4.1 restrict GETATTR fs_location query to the main transport

In the presence of trunking transports, it's helpful to make sure
that during the migration event, the GETATTR for fs_location attribute
happens on the main transport.

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>