bpf: cleanup aux->used_maps after jit

In commit b4ce5923e780 ("bpf, x86: add new map type: instructions array")
env->used_map was copied to func[i]->aux->used_maps before jitting.
Clear these fields out after jitting such that pointer to freed memory
(env->used_maps is freed later) are not kept in a live data structure.

The reason why the copies were initially added is explained in
https://lore.kernel.org/bpf/[e-mail]

Suggested-by: Alexei Starovoitov <[e-mail]>
Fixes: b4ce5923e780 ("bpf, x86: add new map type: instructions array")
Signed-off-by: Anton Protopopov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/verifier.c

···
 		cond_resched();
 	}
 
+	/*
+	 * Cleanup func[i]->aux fields which aren't required
+	 * or can become invalid in future
+	 */
+	for (i = 0; i < env->subprog_cnt; i++) {
+		func[i]->aux->used_maps = NULL;
+		func[i]->aux->used_map_cnt = 0;
+	}
+
 	/* finally lock prog and jit images for all functions and
 	 * populate kallsysm. Begin at the first subprogram, since
 	 * bpf_prog_load will add the kallsyms for the main program.