SUNRPC: correct error code comment in xs_tcp_setup_socket()

This comment was introduced by commit 6ea44adc
("SUNRPC: ensure correct error is reported by xs_tcp_setup_socket()").

I believe EIO was a typo at the time: it should have been EAGAIN.

Subsequently, commit 0445f92c ("SUNRPC: Fix disconnection races")
changed that to ENOTCONN.

Rather than trying to keep the comment here in sync with the code in
xprt_force_disconnect(), make the point in a non-specific way.

Fixes: 6ea44adc

 ("SUNRPC: ensure correct error is reported by xs_tcp_setup_socket()")
Signed-off-by: Calum Mackay <calum.mackay@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>