pNFS: Fix use after free issues in pnfs_do_read()

The assumption should be that if the caller returns PNFS_ATTEMPTED, then hdr
has been consumed, and so we should not be testing hdr->task.tk_status.
If the caller returns PNFS_TRY_AGAIN, then we need to recoalesce and
free hdr.

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