mime/multipart: handle ReadForm(math.MaxInt64) better

Returning an error about integer overflow is needlessly pedantic.
The meaning of ReadForm(MaxInt64) is easily understood
(accept a lot of data) and can be implemented.

Fixes #40430.

Change-Id: I8a522033dd9a2f9ad31dd2ad82cf08d553736ab9
Reviewed-on: https://go-review.googlesource.com/c/go/+/275112


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>