NFS/pnfs: Do not clobber existing pgio_done_cb in nfs4_proc_read_setup

If a pNFS client sets hdr->pgio_done_cb, then we should not overwrite that
in nfs4_proc_read_setup()

Fixes: 75bf47eb

 ("pNFS/flexfile: Fix erroneous fall back to...")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>