UAPI: ethtool: Use __struct_group() in struct ethtool_link_settings

UAPI: ethtool: Use __struct_group() in struct ethtool_link_settings

Use the `__struct_group()` helper to create a new tagged
`struct ethtool_link_settings_hdr`. This structure groups together
all the members of the flexible `struct ethtool_link_settings`
except the flexible array. As a result, the array is effectively
separated from the rest of the members without modifying the memory
layout of the flexible structure.

This new tagged struct will be used to fix problematic declarations
of middle-flex-arrays in composite structs[1].

[1] https://git.kernel.org/linus/d88cabfd9abc

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/9e9fb0bd72e5ba1e916acbb4995b1e358b86a689.1730238285.git.gustavoars@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>