UAPI: net/sched: Use __struct_group() in flex struct tc_u32_sel

UAPI: net/sched: Use __struct_group() in flex struct tc_u32_sel

Use the `__struct_group()` helper to create a new tagged
`struct tc_u32_sel_hdr`. This structure groups together all the
members of the flexible `struct tc_u32_sel` 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/e59fe833564ddc5b2cc83056a4c504be887d6193.1723586870.git.gustavoars@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>