Correct the mask used to get the flow label, since the field is 20 bits, 
not 24 bits.

Found by Dotan Barak and Yaron Gepstein of Mellanox. 
Signed-off-by: NJack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: NRoland Dreier <rolandd@cisco.com>