net: macb: match skb_reserve(skb, NET_IP_ALIGN) with HW alignment

net: macb: match skb_reserve(skb, NET_IP_ALIGN) with HW alignment

If HW is RSC capable, it cannot add dummy bytes at the start of IP
packets. Alignment (ie number of dummy bytes) is configured using the
RBOF field inside the NCFGR register.

On the software side, the skb_reserve(skb, NET_IP_ALIGN) call must only
be done if those dummy bytes are added by the hardware; notice the
skb_reserve() is done AFTER writing the address to the device.

We cannot do the skb_reserve() call BEFORE writing the address because
the address field ignores the low 2/3 bits. Conclusion: in some cases,
we risk not being able to respect the NET_IP_ALIGN value (which is
picked based on unaligned CPU access performance).

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20251023-macb-eyeq5-v3-2-af509422c204@bootlin.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>