NFSv4: Treat ENETUNREACH errors as fatal for state recovery

If a containerised process is killed and causes an ENETUNREACH or
ENETDOWN error to be propagated to the state manager, then mark the
nfs_client as being dead so that we don't loop in functions that are
expecting recovery to succeed.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>