random: fix BUG_ON caused by accounting simplification

Commit ee1de406 ("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: N"Theodore Ts'o" <tytso@mit.edu>
Reported-by: NPeter Zijlstra <peterz@infradead.org>
Reported-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Greg Price <price@mit.edu>

drivers/char/random.c

@@ -995,8 +995,11 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
 		ibytes = min_t(size_t, ibytes, have_bytes - reserved);
 	if (ibytes < min)
 		ibytes = 0;
-	entropy_count = max_t(int, 0,
-			      entropy_count - (ibytes << (ENTROPY_SHIFT + 3)));
+	if (have_bytes >= ibytes + reserved)
+		entropy_count -= ibytes << (ENTROPY_SHIFT + 3);
+	else
+		entropy_count = reserved << (ENTROPY_SHIFT + 3);
+
 	if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
 		goto retry;