NFS/localio: Fix a race in nfs_local_open_fh()

Once the clp->cl_uuid.lock has been dropped, another CPU could come in
and free the struct nfsd_file that was just added. To prevent that from
happening, take the RCU read lock before dropping the spin lock.

Fixes: 86e00412 ("nfs: cache all open LOCALIO nfsd_file(s) in client")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>