Revert "random: block in /dev/urandom"

This reverts commit 6f98a4bf

.

It turns out we still can't do this.  Way too many platforms that don't
have any real source of randomness at boot and no jitter entropy because
they don't even have a cycle counter.

As reported by Guenter Roeck:

 "This causes a large number of qemu boot test failures for various
  architectures (arm, m68k, microblaze, sparc32, xtensa are the ones I
  observed).

  Common denominator is that boot hangs at 'Saving random seed:'"

This isn't hugely unexpected - we tried it, it failed, so now we'll
revert it.

Link: https://lore.kernel.org/all/20220322155820.GA1745955@roeck-us.net/
Reported-and-bisected-by: Guenter Roeck <linux@roeck-us.net>
Cc: Jason Donenfeld <Jason@zx2c4.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>