NFS: Treat ENETUNREACH errors as fatal in containers

Propagate the NFS_MOUNT_NETUNREACH_FATAL flag to work with the generic
NFS client. If the flag is set, the client will receive ENETDOWN and
ENETUNREACH errors from the RPC layer, and is expected to treat them as
being fatal.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Chuck Lever <chuck.lever@oracle.com>