Merge branch 'introduce-define_flex-macro'

Merge branch 'introduce-define_flex-macro'

Przemek Kitszel says:

====================
introduce DEFINE_FLEX() macro

Add DEFINE_FLEX() macro, that helps on-stack allocation of structures
with trailing flex array member.
Expose __struct_size() macro which reads size of data allocated
by DEFINE_FLEX().

Accompany new macros introduction with actual usage,
in the ice driver - hence targeting for netdev tree.

Obvious benefits include simpler resulting code, less heap usage,
less error checking. Less obvious is the fact that compiler has
more room to optimize, and as a whole, even with more stuff on the stack,
we end up with overall better (smaller) report from bloat-o-meter:
add/remove: 8/6 grow/shrink: 7/18 up/down: 2211/-2270 (-59)
(individual results in each patch).
====================

Link: https://lore.kernel.org/r/20230912115937.1645707-1-przemyslaw.kitszel@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>