nfp: fix free list buffer size reporting

XDP headroom should not be included in free list buffer size.

Fixes: 6fe0c3b4

 ("nfp: add support for xdp_adjust_head()")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>