net: dsa: microchip: linearize skb for tail-tagging switches

The pointer arithmentic for accessing the tail tag only works
for linear skbs.

For nonlinear skbs, it reads uninitialized memory inside the
skb headroom, essentially randomizing the tag. I have observed
it gets set to 6 most of the time.

Example where ksz9477_rcv thinks that the packet from port 1 comes from port 6
(which does not exist for the ksz9896 that's in use), dropping the packet.
Debug prints added by me (not included in this patch):

[ 256.645337] ksz9477_rcv:323 tag0=6
[ 256.645349] skb len=47 headroom=78 headlen=0 tailroom=0
mac=(64,14) mac_len=14 net=(78,0) trans=78
shinfo(txflags=0 nr_frags=1 gso(size=0 type=0 segs=0))
csum(0x0 start=0 offset=0 ip_summed=0 complete_sw=0 valid=0 level=0)
hash(0x0 sw=0 l4=0) proto=0x00f8 pkttype=1 iif=3
priority=0x0 mark=0x0 alloc_cpu=0 vlan_all=0x0
encapsulation=0 inner(proto=0x0000, mac=0, net=0, trans=0)
[ 256.645377] dev name=end1 feat=0x0002e10200114bb3
[ 256.645386] skb headroom: 00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 256.645395] skb headroom: 00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 256.645403] skb headroom: 00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 256.645411] skb headroom: 00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 256.645420] skb headroom: 00000040: ff ff ff ff ff ff 00 1c 19 f2 e2 db 08 06
[ 256.645428] skb frag: 00000000: 00 01 08 00 06 04 00 01 00 1c 19 f2 e2 db 0a 02
[ 256.645436] skb frag: 00000010: 00 83 00 00 00 00 00 00 0a 02 a0 2f 00 00 00 00
[ 256.645444] skb frag: 00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01
[ 256.645452] ksz_common_rcv:92 dsa_conduit_find_user returned NULL

Call skb_linearize before trying to access the tag.

This patch fixes ksz9477_rcv which is used by the ksz9896 I have at
hand, and also applies the same fix to ksz8795_rcv which seems to have
the same problem.

Signed-off-by: Jakob Unterwurzacher <jakob.unterwurzacher@cherry.de>
CC: stable@vger.kernel.org
Fixes: 016e43a26bab ("net: dsa: ksz: Add KSZ8795 tag code")
Fixes: 8b8010fb7876 ("dsa: add support for Microchip KSZ tail tagging")
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/20250515072920.2313014-1-jakob.unterwurzacher@cherry.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>