random: simplify accounting logic

This logic is exactly equivalent to the old logic, but it should
be easier to see what it's doing.

The equivalence depends on one fact from outside this function:
when 'r->limit' is false, 'reserved' is zero.  (Well, two facts;
the other is that 'reserved' is never negative.)

Cc: Jiri Kosina <jkosina@suse.cz>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Greg Price <price@mit.edu>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

drivers/char/random.c

@@ -984,14 +984,10 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
 		ibytes = 0;
 	} else {
 		/* If limited, never pull more than available */
-		if (r->limit && ibytes + reserved >= have_bytes)
-			ibytes = have_bytes - reserved;
-
-		if (have_bytes >= ibytes + reserved)
-			entropy_count -= ibytes << (ENTROPY_SHIFT + 3);
-		else
-			entropy_count = reserved << (ENTROPY_SHIFT + 3);
-
+		if (r->limit)
+			ibytes = min_t(size_t, ibytes, have_bytes - reserved);
+		entropy_count = max_t(int, 0,
+			    entropy_count - (ibytes << (ENTROPY_SHIFT + 3)));
 		if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
 			goto retry;