net: generalize calculation of skb extensions length

Remove the necessity to modify skb_ext_total_length() when new extension
types are added.
Also reduces the line count a bit.

With optimizations enabled the function is folded down to the same
constant value as before during compilation.
This has been validated on x86 with GCC 6.5.0 and 13.2.1.
Also a similar construct has been validated on godbolt.org with GCC 5.1.
In any case the compiler has to be able to evaluate the construct at
compile-time for the BUILD_BUG_ON() in skb_extensions_init().

Even if not evaluated at compile-time this function would only ever
be executed once at run-time, so the overhead would be very minuscule.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20230823-skb_ext-simplify-v2-1-66e26cd66860@weissschuh.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>