nfp: prepare metadata handling for xdp_adjust_head()

XDP may require us to move metadata to make room for pushing
headers.  Track meta data location with a pointer and pass
it explicitly to functions.

While at it validate that meta_len from the descriptor is not
bogus.

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