random: fix BUG_ON caused by accounting simplification

random: fix BUG_ON caused by accounting simplification

Commit ee1de406ba6eb1 ("random: simplify accounting logic") simplified
things too much, in that it allows the following to trigger an
overflow that results in a BUG_ON crash:

dd if=/dev/urandom of=/dev/zero bs=67108707 count=1

Thanks to Peter Zihlstra for discovering the crash, and Hannes
Frederic for analyizing the root cause.

Signed-off-by: "Theodore Ts'o" <[email protected]>
Reported-by: Peter Zijlstra <[email protected]>
Reported-by: Hannes Frederic Sowa <[email protected]>
Cc: Greg Price <[email protected]>