vlan: ensure sufficient headroom in vlan_dev_hard_header()

Callers that only reserve ETH_HLEN or less (such as llc_alloc_frame()),
or skbs allocated before dynamic device/headroom changes (e.g. toggling
VLAN_FLAG_REORDER_HDR or bonding/team switching slaves), can reach
vlan_dev_hard_header() with insufficient headroom and trigger
skb_under_panic().

Use skb_cow_head() in vlan_dev_hard_header() when VLAN_FLAG_REORDER_HDR
is not set to ensure sufficient headroom for the VLAN header(s) and the
underlying device hard header.

Use READ_ONCE() to read dev->hard_header_len and dev->needed_headroom as
they can be updated concurrently under RTNL (e.g. in
vlan_transfer_features()) while vlan_dev_hard_header() runs locklessly on
the transmit path. Also avoid LL_RESERVED_SPACE(dev) here so that the
extra HH_DATA_MOD alignment padding does not trigger unnecessary
pskb_expand_head() reallocations on inner stacked VLAN devices after the
outer VLAN header has been pushed.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Zixuan Chai <petalzu987@gmail.com>
Closes: https://lore.kernel.org/netdev/cover.1789987105.git.petalzu987@gmail.com/
Link: https://lore.kernel.org/netdev/179022851638.2160803.1808206741379444999@kernel.org/
Cc: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260924082951.1599377-5-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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