random: make 'add_interrupt_randomness()' do something sane

We've been moving away from add_interrupt_randomness() for various
reasons: it's too expensive to do on every interrupt, and flooding the
CPU with interrupts could theoretically cause bogus floods of entropy
from a somewhat externally controllable source.

This solves both problems by limiting the actual randomness addition
to just once a second or after 64 interrupts, whicever comes first.
During that time, the interrupt cycle data is buffered up in a per-cpu
pool.  Also, we make sure the the nonblocking pool used by urandom is
initialized before we start feeding the normal input pool.  This
assures that /dev/urandom is returning unpredictable data as soon as
possible.

(Based on an original patch by Linus, but significantly modified by
tytso.)

Tested-by: Eric Wustrow <[email protected]>
Reported-by: Eric Wustrow <[email protected]>
Reported-by: Nadia Heninger <[email protected]>
Reported-by: Zakir Durumeric <[email protected]>
Reported-by: J. Alex Halderman <[email protected]>.
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: "Theodore Ts'o" <[email protected]>
Cc: [email protected]

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