NFS: Don't check lock owner compatability unless file is locked (part 2)

When coalescing requests into a single READ or WRITE RPC call, and there
is no file locking involved, we don't have to refuse coalescing for
requests where the lock owner information doesn't match.

Reported-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>