nfp: add support for xdp_adjust_head()

Support prepending data from XDP.  We are already always allocating
some headroom because FW may prepend metadata to packets.
xdp_adjust_head() can be supported by making sure that headroom is
big enough for XDP.  In case FW had prepended metadata to the packet,
however, we have to move it out of the way before we call XDP.

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