Always send IO to OST_IO_PORTAL regardless of what the default portal is.
Was previously broken for echo_client on catamount, but would also break
on 1.8 once the default request portal is REQPORTAL.

Detect if OST_WRITE or OST_READ requests go to non-IO portals, as that
would LBUG the server due to lack of IO buffers.
b=11813
i=green
i=shadow