eth: fbnic: Update Headroom

eth: fbnic: Update Headroom

Fbnic currently reserves a minimum of 64B headroom, but this is
insufficient for inserting additional headers (e.g., IPV6) via XDP, as
only 24 bytes are available for adjustment. To address this limitation,
increase the headroom to a larger value while ensuring better page use.
Although the resulting headroom (192B) is smaller than the recommended
value (256B), forcing the headroom to 256B would require aligning to
256B (as opposed to the current 128B), which can push the max headroom
to 511B.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Link: https://patch.msgid.link/20250813221319.3367670-3-mohsin.bashr@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>