nfsd4: maintain one seqid stream per (lockowner, file)

Instead of creating a new lockowner and stateid for every
open_to_lockowner call, reuse the existing lockowner if it exists.

Reported-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>