The arch_get_random_seed_long() invocation done by the random device
driver is done in interrupt context and may be invoked very very
frequently. The existing s390 arch_get_random_seed*() implementation
uses the PRNO(TRNG) instruction which produces excellent high quality
entropy but is relatively slow and thus expensive.

This fix reworks the arch_get_random_seed* implementation. It
introduces a buffer concept to decouple the delivery of random data
via arch_get_random_seed*() from the generation of new random
bytes. The buffer of random data is filled asynchronously by a
workqueue thread.
If there are enough bytes in the buffer the s390_arch_random_generate()
just delivers these bytes. Otherwise false is returned until the worker
thread refills the buffer.
The worker fills the rng buffer by pulling fresh entropy from the
high quality (but slow) true hardware random generator. This entropy
is then spread over the buffer with an pseudo random generator.
...