net: dsa: tag_brcm: legacy FCS: request needed tailroom

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

net: dsa: tag_brcm: legacy FCS: request needed tailroom

The legacy FCS tagger calculates the CRC over skb->len bytes starting at
skb->data. When a nonlinear skb reaches the tagger, this reads past the
linear head into unrelated slab memory.

The tagger appends an Ethernet FCS but does not declare that tailroom. As a
result, DSA leaves NETIF_F_SG and NETIF_F_FRAGLIST enabled on the user
port, and nonlinear skbs can reach the CRC calculation.

Declare the required tailroom. DSA will then clear those features and the
networking core will linearize skbs before the tagger runs.

A KASAN-enabled dsa_loop test using this tagger reports:

BUG: KASAN: slab-out-of-bounds in crc32_le
Read of size 1 at addr ffff8880397086c0 by task exp/135

Call Trace:
crc32_le (lib/crc/crc32-main.c:38)
brcm_leg_fcs_tag_xmit (net/dsa/tag_brcm.c:343)
dsa_user_xmit (net/dsa/user.c:942)
dev_hard_start_xmit (net/core/dev.c:3937)
__dev_queue_xmit (net/core/dev.c:4926)
packet_sendmsg (net/packet/af_packet.c:3110)
__sys_sendto (net/socket.c:2281)

The buggy address belongs to the object at ffff888039708400
which belongs to the cache skbuff_small_head of size 704
The buggy address is located 0 bytes to the right of
allocated 704-byte region [ffff888039708400, ffff8880397086c0)