vxlan: use one headroom snapshot for neighbour replies

vxlan_na_create() samples LL_RESERVED_SPACE() to size the reply skb and then
samples it again to reserve headroom. A concurrent vxlan_changelink() can
update needed_headroom between the two reads, creating a TOCTOU race. The
second value can exceed the allocation and make the Ethernet header write out
of bounds.

The race is reproducible on the unpatched kernel. It occurred when
vxlan_na_create() generated a neighbour reply while vxlan_changelink() changed
the link headroom. KASAN caught a four-byte write two bytes beyond a 704-byte
skbuff_small_head allocation.

Snapshot the headroom once and use that value for both allocation and
reservation.

Fixes: 4b29dba9c085 ("vxlan: fix nonfunctional neigh_reduce()")
Signed-off-by: Sanghyun Park <sanghyun.park.cnu@gmail.com>
Link: https://patch.msgid.link/20260918032842.502409-2-sanghyun.park.cnu@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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