nios2: use fallback for random_get_entropy() instead of zero

commit c04e727 upstream.

In the event that random_get_entropy() can't access a cycle counter or
similar, falling back to returning 0 is really not the best we can do.
Instead, at least calling random_get_entropy_fallback() would be
preferable, because that always needs to return _something_, even
falling back to jiffies eventually. It's not as though
random_get_entropy_fallback() is super high precision or guaranteed to
be entropic, but basically anything that's not zero all the time is
better than returning zero all the time.

Cc: Thomas Gleixner <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Acked-by: Dinh Nguyen <[email protected]>
Signed-off-by: Jason A. Donenfeld <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: zx2c4

arch/nios2/include/asm/timex.h

@@ -8,5 +8,8 @@
 typedef unsigned long cycles_t;
 
 extern cycles_t get_cycles(void);
+#define get_cycles get_cycles
+
+#define random_get_entropy() (((unsigned long)get_cycles()) ?: random_get_entropy_fallback())
 
 #endif