x86/espfix: Use get_random_long() rather than archrandom

A call is made to arch_get_random_longs() and rdtsc(), rather than just
using get_random_long(), because this was written during a time when
very early boot would give abysmal entropy. These days, a call to
get_random_long() at early boot will incorporate RDRAND, RDTSC, and
more, without having to do anything bespoke.

In fact, the situation is now such that on the majority of x86 systems,
the pool actually is initialized at this point, even though it doesn't
need to be for get_random_long() to still return something better than
what this function currently does.

So simplify this to just call get_random_long() instead.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221029002613.143153-1-Jason@zx2c4.com

Author: zx2c4

arch/x86/kernel/espfix_64.c

@@ -94,17 +94,7 @@ static inline unsigned long espfix_base_addr(unsigned int cpu)
 
 static void init_espfix_random(void)
 {
-	unsigned long rand;
-
-	/*
-	 * This is run before the entropy pools are initialized,
-	 * but this is hopefully better than nothing.
-	 */
-	if (!arch_get_random_longs(&rand, 1)) {
-		/* The constant is an arbitrary large prime */
-		rand = rdtsc();
-		rand *= 0xc345c6b72fd16123UL;
-	}
+	unsigned long rand = get_random_long();
 
 	slot_random = rand % ESPFIX_STACKS_PER_PAGE;
 	page_random = (rand / ESPFIX_STACKS_PER_PAGE)