nfs: initialize the backing_dev_info when creating the server

NFS may free the server structure without ever having used the
bdi, so we either need to flag the bdi as being uninitialized or
initialize it up front. This does the latter.

This fixes a crash with mounting more than one NFS file system,
should people ever need that kind of obscure NFS functionality.

Tested-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>