lib/dpif-netdev: Fix EMC lookup.

Patch 0de8783a9 (lib/dpif-netdev: Integrate megaflow classifier.)
broke exact match cache lookup, but it went undetected since there are
no separate stats for EMC.

This patch fixes the problem by changing the struct netdev_flow_key
'len' member to cover only the 'mf' member, not the whole
netdev_flow_key, and ignoring the 'len' field in
netdev_flow_key_equal.  Comparison is still accurate, as the miniflow
'map' field encodes the length in the number of 1-bits, and the map is
included in the comparison.

Reported-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Daniele Di Proietto <ddiproietto@vmware.com>