net: stmmac: restore NET_IP_ALIGN in the RX DMA offset

In the Linux kernel, the following vulnerability has been resolved:

net: stmmac: restore NET_IP_ALIGN in the RX DMA offset

Since the RX path was converted to zero-copy, the page pool page is handed
to the stack directly as the skb head, and the offset the DMA engine writes
at is what determines the alignment of the packet headers.

Before the conversion the payload was copied into an skb obtained from
napi_alloc_skb(), which reserves NET_SKB_PAD + NET_IP_ALIGN. The
conversion moved the headroom into stmmac_rx_offset() but did not carry
over NET_IP_ALIGN, so on architectures where NET_IP_ALIGN is 2 the IP
header now lands misaligned:

64 (NET_SKB_PAD) + 14 (ethernet) + 20 (IP) = 98

Same for the XDP branch:

256 (XDP_PACKET_HEADROOM) + 14 (ethernet) + 20 (IP) = 290

On ARM32 this is fatal, because ldm and ldrd trap on unaligned addresses
even when CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is set.

Any received echo request panics the machine, e.g:

Unhandled fault: alignment exception (0x001) at 0x81873062
Internal error: : 1 [#1] SMP ARM
Hardware name: Altera SOCFPGA Arria10
PC is at icmp_echo+0x38/0xa8
LR is at icmp_rcv+0x22c/0x370
Call trace:
icmp_echo from icmp_rcv+0x22c/0x370
icmp_rcv from ip_protocol_deliver_rcu+0x2c/0x224
ip_protocol_deliver_rcu from ip_local_deliver+0xc8/0x1a0
ip_local_deliver from ip_sublist_rcv_finish+0x3c/0x50
ip_sublist_rcv_finish from ip_list_rcv_finish+0x110/0x118
ip_list_rcv_finish from ip_list_rcv+0xc8/0xdc
ip_list_rcv from __netif_receive_skb_list_core+0x170/0x1c0
...
napi_complete_done from stmmac_napi_poll_rx+0xcb0/0x1030
Code: e24dd068 e59020a0 e28dc010 e0822001 (e8920003)
Kernel panic - not syncing: Fatal exception in interrupt

The faulting instruction is the ldm of *icmp_hdr(skb) in icmp_echo().

Fix by adding NET_IP_ALIGN back to the RX offset, which restores the
alignment the stack used to get.

Note that commit a955318fe67e ("stmmac: align RX buffers") made a similar
change in 2021 and was reverted by commit 12d125b4574b ("stmmac: Revert
"stmmac: align RX buffers"") because it caused packet corruption. That
patch raised the offset from 0 without adjusting the buffer size
accounting, so the DMA engine could arguably write past the end of the RX
buffers, though this was never root caused.
Commit df542f669307 ("net: stmmac: Switch to zero-copy in non-XDP RX
path") since derives the page pool allocation from stmmac_rx_offset(), so
the extra bytes are accounted for.