random: make backtracking attacks harder

At each extraction, we change (poolbits / 16) + 32 bits in the pool,
or 96 bits in the case of the secondary pools. Thus, a brute-force
backtracking attack on the pool state is less difficult than breaking
the hash. In certain cases, this difficulty may be is reduced to 2^64
iterations.

Instead, hash the entire pool in one go, then feedback the whole hash
(160 bits) in one go. This will make backtracking at least as hard as
inverting the hash.

Signed-off-by: Matt Mackall <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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