flow: Define miniflow_get__() to simplify code.

miniflow_get__() can be used when it is known that the miniflow stores
a value at the given index.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
flow.c: Improve minimask_equal() and minimask_has_extra().

minimask_equal() and minimask_has_extra() can take benefit from the
fact that minimasks have no zero data.

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