NFSv4: nfs4_do_open() is incorrectly triggering state recovery

We're seeing spurious calls to nfs4_schedule_stateid_recovery() from
nfs4_do_open() in situations where there is no trigger coming from the
server.
In theory the code path being triggered is supposed to notice that state
recovery happened while we were processing the open call result from the
server, before the open stateid is published. However in the years since
that code was added, we've also added the 'session draining' mechanism,
which ensures that the state recovery will wait until all the session
slots have been returned. In nfs4_do_open() the session slot is only
returned on exit of the function, so we don't need the legacy mechanism.

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