random: clear new batches when bringing new CPUs online

The commit that added the new get_random_{u8,u16}() functions neglected
to update the code that clears the batches when bringing up a new CPU.
It also forgot a few comments and helper defines, so add those in too.

Fixes: 585cd5fe ("random: add 8-bit and 16-bit batches")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>