lib/flow.h Revert bitfield back to uint64_t.

Using different types for the two bitfields did not work on MSVC, so
reverting back to "64-bit bool" :-)

Reported-by: Saurabh Shah <ssaurabh@vmware.com>
Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>