Optimize crc32 computation

Move the crc table initialization outside of the loop to avoid to check the
init flag for every byte.

My benchmark show a gain of 26% to clone a partition of 17G with 10G used:

init crc32 table inside the loop:
real   4m23.649s
user   2m20.301s
sys    1m34.782s

init crc32 table outside the loop:
real   3m13.169s
user   1m15.269s
sys    1m30.966s

Signed-off-by: Patryck Rouleau <pfrouleau@gmail.com>