tap, pasta: Handle incomplete tap sends for pasta too

Since a469fc39 ("tcp, tap: Don't increase tap-side sequence counter for
dropped frames") we've handled more gracefully the case where we get data
from the socket side, but are temporarily unable to send it all to the tap
side (e.g. due to full buffers).

That code relies on tap_send_frames() returning the number of frames it
successfully sent, which in turn gets it from tap_send_frames_passt() or
tap_send_frames_pasta().

While tap_send_frames_passt() has returned that information since b62ed9ca
("tap: Don't pcap frames that didn't get sent"), tap_send_frames_pasta()
always returns as though it succesfully sent every frame.  However there
certainly are cases where it will return early without sending all frames.
Update it report that properly, so that the calling functions can handle it
properly.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tap.c

@@ -330,8 +330,6 @@ static size_t tap_send_frames_pasta(const struct ctx *c,
 			case EWOULDBLOCK:
 #endif
 			case EINTR:
-				i--;
-				break;
 			case ENOBUFS:
 			case ENOSPC:
 				break;
@@ -341,7 +339,7 @@ static size_t tap_send_frames_pasta(const struct ctx *c,
 		}
 	}
 
-	return n;
+	return i;
 }
 
 /**