hw_random carefully avoids using a stack buffer except in
add_early_randomness().  This causes a crash in virtio_rng if
CONFIG_VMAP_STACK=y.
Reported-by: NMatt Mullins <mmullins@mmlx.us>
Tested-by: NMatt Mullins <mmullins@mmlx.us>
Fixes: d3cc7996 ("hwrng: fetch randomness only after device init")
Signed-off-by: NAndy Lutomirski <luto@kernel.org>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>