Instead of "inconsistent poll.fdMutex", panic with
"too many concurrent operations on a single file or socket (max 1048575)".

Fixes #25558

Change-Id: I5cad3633aa539fb6f48cca236c6656c86acfb663
Reviewed-on: https://go-review.googlesource.com/119956
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>