nfscl: Enable support for the Lookup+Open RPC

Commits 3ad1e1c1ce20 and 57014f21e754 added a Lookup+Open
RPC for NFSv4.1/4.2, which can reduce the RPC count by
10-20% for some loads. This has now received a fair amount
of testing, so I think it is ok to enable it.

Note that the Lookup+Open RPC is only used when the
"oneopenown" mount option is specified. As such, this
change won't affect most NFSv4.1/4.2 mounts.