bpf, sockmap: Don't let sock_map_{close,destroy,unhash} call itself

In the Linux kernel, the following vulnerability has been resolved: bpf, sockmap: Don't let sock_map_{close,destroy,unhash} call itself sock_map proto callbacks should never call themselves by design. Protect against bugs like [1] and break out of the recursive loop to avoid a stack overflow in favor of a resource leak. [1] https://lore.kernel.org/all/[email protected]/