fs/netfs/read_collect: add to next->prev_donated

If multiple subrequests donate data to the same "next" request
(depending on the subrequest completion order), each of them would
overwrite the `prev_donated` field, causing data corruption and a
BUG() crash ("Can't donate prior to front").

Fixes: ee4cdf7 ("netfs: Speed up buffered reading")
Closes: https://lore.kernel.org/netfs/CAKPOu+_4mUwYgQtRTbXCmi+-k3PGvLysnPadkmHOyB7Gz0iSMA@mail.gmail.com/
Cc: stable@vger.kernel.org
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: MaxKellermann

fs/netfs/read_collect.c

@@ -284,7 +284,7 @@ static bool netfs_consume_read_data(struct netfs_io_subrequest *subreq, bool was
 				   netfs_trace_donate_to_deferred_next);
 	} else {
 		next = list_next_entry(subreq, rreq_link);
-		WRITE_ONCE(next->prev_donated, excess);
+		WRITE_ONCE(next->prev_donated, next->prev_donated + excess);
 		trace_netfs_donate(rreq, subreq, next, excess,
 				   netfs_trace_donate_to_next);
 	}