netflow: Back-out optimization that could lead to infinite loop

netflow: Back-out optimization that could lead to infinite loop

Commit 924282 (netflow: Do 64-bit division less often.) attempted to
remove the 64-bit division used to break flow records with large byte
counts into multiple NetFlow records.  The calculation to determine the
number of records was incorrect and should have shifted "byte_delta" by
31 instead of 32.  This commit reverts the change (while keeping commit
f22a24 (netflow: Avoid (theoretically) looping 2**32 times.) ), since
the logic is more straight-forward than the optimized version.

Bug #3813