The linux getrandom system call returns at most 33554431 = 2^25-1 bytes per
call. The existing behavior for larger reads is to report a failure, because
there appears to have been an unexpected short read. In this case the system
falls back to reading from "/dev/urandom".

This change performs reads of 2^25 bytes or more with multiple calls to
getrandom.

Fixes #20877

Change-Id: I618855bdedafd86cd11219fe453af1d6fa2c88a7
Reviewed-on: https://go-review.googlesource.com/49170Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>