hwrng: core - move add_early_randomness() out of rng_mutex

add_early_randomness() is called every time a new rng backend is added
and every time it is set as the current rng provider.

add_early_randomness() is called from functions locking rng_mutex,
and if it hangs all the hw_random framework hangs: we can't read sysfs,
add or remove a backend.

This patch move add_early_randomness() out of the rng_mutex zone.
It only needs the reading_mutex.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>