pNFS: Return RW layouts on OPEN_DOWNGRADE

If the client holds no more writeable open state, and does not hold a
write delegation, then send a layoutreturn as part of the OPEN_DOWNGRADE.

We do this only for writes, since some layout drivers may require you to
also hold a read layout if you are doing a R/W workload.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>