net: fec: add change_mtu to support dynamic buffer allocation

Add a fec_change_mtu() handler to recalculate the pagepool_order based on
the new_mtu value. And update the rx_frame_size accordingly when
pagepool_order changes.

MTU changes are only allowed when the adater is not running.

Reviewed-by: Wei Fang <wei.fang@nxp.com>
Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
Link: https://patch.msgid.link/20250910185211.721341-6-shenwei.wang@nxp.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>