pnfs: allow LD to ask to resend read through pnfs

If current IO cannot be completed due to some transient errors,
LD may want to ask generic layer to resend the request through
pnfs again.

Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Tom Haynes <loghyr@primarydata.com>