pNFS/flexfiles: Treat ENETUNREACH errors as fatal in containers

Propagate the NFS_MOUNT_NETUNREACH_FATAL flag to work with the pNFS
flexfiles client. In these circumstances, the client needs to treat the
ENETDOWN and ENETUNREACH errors as fatal, and should abandon the
attempted I/O.

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>