net: pppoe: avoid zero-length arrays in struct pppoe_hdr

net: pppoe: avoid zero-length arrays in struct pppoe_hdr

Jakub Kicinski reported following issue in upcoming patches:

W=1 C=1 GCC build gives us:

net/bridge/netfilter/nf_conntrack_bridge.c: note: in included file (through
../include/linux/if_pppox.h, ../include/uapi/linux/netfilter_bridge.h,
../include/linux/netfilter_bridge.h): include/uapi/linux/if_pppox.h:
153:29: warning: array of flexible structures

sparse doesn't like that hdr has a zero-length array which overlaps
proto. The kernel code doesn't currently need those arrays.

PPPoE connection is functional after applying this patch.

Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Eric Woudstra <ericwouds@gmail.com>
Link: https://patch.msgid.link/20260224155030.106918-1-ericwouds@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>