flow: Clean up MINIFLOW_FOR_EACH_IN_MAP.

It seemed awkward to have declarations outside the for loop.

This may also be a little faster because it avoids some calls to
count_1bits().  The idea for that change is due to Jarno Rajahalme
<jrajahalme@nicira.com>.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>