arm: use fallback for random_get_entropy() instead of zero

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 <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Reviewed-by: Russell King (Oracle) <[e-mail]>
Signed-off-by: Jason A. Donenfeld <[e-mail]>

arch/arm/include/asm/timex.h

···
 
 typedef unsigned long cycles_t;
 #define get_cycles()	({ cycles_t c; read_current_timer(&c) ? 0 : c; })
+#define random_get_entropy() (((unsigned long)get_cycles()) ?: random_get_entropy_fallback())
 
 #endif