sfc: Add XDP frame size

This driver uses RX page-split when possible. It was recently fixed
in commit 86e85bf6 ("sfc: fix XDP-redirect in this driver") to
add needed tailroom for XDP-redirect.

After the fix efx->rx_page_buf_step is the frame size, with enough
head and tail-room for XDP-redirect.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/158945335278.97035.14611425333184621652.stgit@firesoul