crypto/rsa: ensure that generating toy RSA keys doesn't loop.

crypto/rsa: ensure that generating toy RSA keys doesn't loop.

If there are too few primes of the given length then it can be
impossible to generate an RSA key with n distinct primes.

This change approximates the expected number of candidate primes and
causes key generation to return an error if it's unlikely to succeed.

Fixes #16596.

Change-Id: I53b60d0cb90e2d0e6f0662befa64d13f24af51a7
Reviewed-on: https://go-review.googlesource.com/28969
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>