dedupe: fixing bug with subsequent dedupe buffer generation

dedupe: fixing bug with subsequent dedupe buffer generation

When unique pages are generated onto the write buffer the seed
used to generate the page is copied later to the buf_state_prev
for future dedupe generation.

However, the get_buf_state API returns pointer to the prev
seed. Then when the caller uses it to re-generate the buffer
it changes the internal seed and advances the PRNG.
A subsequent intention to create another dedup might result in
generating a unique page instead.

Signed-off-by: Bar David <bardavvid@gmail.com>