NFSv4.1: Close callback races for OPEN, LAYOUTGET and LAYOUTRETURN

Defer freeing the slot until after we have processed the results from
OPEN and LAYOUTGET. This means that the server can rely on the
mechanism in RFC5661 Section 2.10.6.3 to ensure that replies to an
OPEN or LAYOUTGET/RETURN RPC call don't race with the callbacks that
apply to them.

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