Not need to create pipes for dying client. So just skip them.

Note: we can safely dereference the client structure, because notification
caller is holding sn->pipefs_sb_lock.
Signed-off-by: NStanislav Kinsbursky <skinsbursky@parallels.com>
Cc: stable@vger.kernel.org
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>