SUNRPC: Use the ENOTCONN error on socket disconnect

When the socket is closed, we currently send an EAGAIN error to all
pending requests in order to ask them to retransmit. Use ENOTCONN
instead, to ensure that they try to reconnect before attempting to
transmit.
This also helps SOFTCONN tasks to behave correctly in this
situation.

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