ipv4/igmp: Annotate struct ip_sf_socklist with __counted_by

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 ip_sf_socklist.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Martin KaFai Lau <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Reviewed-by: Gustavo A. R. Silva <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/linux/igmp.h

···
 	unsigned int		sl_max;
 	unsigned int		sl_count;
 	struct rcu_head		rcu;
-	__be32			sl_addr[];
+	__be32			sl_addr[] __counted_by(sl_max);
 };
 
 #define IP_SFBLOCK	10	/* allocate this many at once */