lib/flow: Simplify miniflow accessors, add ipv6 support.

Add new macro MINIFLOW_MAP(FIELD) that returns the map covering the
given struct flow field.

Change the miniflow accessors to macros so that they can take the
field name directly.

Use these to add ipv6 suppoort to miniflow_hash_5tuple().

Add ipv6 support to flow_hash_5tuple() as well so that these two
functions continue to return the same hash value for the corresponding
flows.

Also, simplify miniflow_get_metadata().

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>