random: mix in architectural randomness in extract_buf()

random: mix in architectural randomness in extract_buf()

Mix in any architectural randomness in extract_buf() instead of
xfer_secondary_buf().  This allows us to mix in more architectural
randomness, and it also makes xfer_secondary_buf() faster, moving a
tiny bit of additional CPU overhead to process which is extracting the
randomness.

[ Commit description modified by tytso to remove an extended
  advertisement for the RDRAND instruction. ]

Signed-off-by: H. Peter Anvin <[email protected]>
Acked-by: Ingo Molnar <[email protected]>
Cc: DJ Johnston <[email protected]>
Signed-off-by: Theodore Ts'o <[email protected]>
Cc: [email protected]