Use arch_get_random_seed*() in two places in the Linux random
driver (drivers/char/random.c):

1. During entropy pool initialization, use RDSEED in favor of RDRAND,
   with a fallback to the latter.  Entropy exhaustion is unlikely to
   happen there on physical hardware as the machine is single-threaded
   at that point, but could happen in a virtual machine.  In that
   case, the fallback to RDRAND will still provide more than adequate
   entropy pool initialization.

2. Once a second, issue RDSEED and, if successful, feed it to the
   entropy pool.  To ensure an extra layer of security, only credit
   half the entropy just in case.
Suggested-by: NLinus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: NIngo Molnar <mingo@kernel.org>
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>