Add code that sets the session defunct after a "soft" mount has returned
an error without completing the RPC.
Without this, if a DS server is down for a long time, RPCs will use up
all the session slots and then processes will hang waiting for a slot.
Once an RPC has returned error without completion, the state of the session
slot is unknown, so setting it defunct seems to be the only option.
Also, add EIO to ENXIO for the case where a DS replies NFSERR_IO to report
I/O problems on the server.