Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct ip6_sf_socklist.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocciSigned-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230922172858.3822653-3-keescook@chromium.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>