nfp: pass packet pointer to nfp_net_parse_meta()

Make nfp_net_parse_meta() take a packet pointer and return
a drop/no drop decision. Right now it returns the end of
metadata and caller compares it to the packet pointer.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>