bpf: Fix memory leaks in __check_func_call

kmemleak reports this issue:

unreferenced object 0xffff88817139d000 (size 2048):
comm "test_progs", pid 33246, jiffies 4307381979 (age 45851.820s)
hex dump (first 32 bytes):
01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
backtrace:
[<0000000045f075f0>] kmalloc_trace+0x27/0xa0
[<0000000098b7c90a>] __check_func_call+0x316/0x1230
[<00000000b4c3c403>] check_helper_call+0x172e/0x4700
[<00000000aa3875b7>] do_check+0x21d8/0x45e0
[<000000001147357b>] do_check_common+0x767/0xaf0
[<00000000b5a595b4>] bpf_check+0x43e3/0x5bc0
[<0000000011e391b1>] bpf_prog_load+0xf26/0x1940
[<0000000007f765c0>] __sys_bpf+0xd2c/0x3650
[<00000000839815d6>] __x64_sys_bpf+0x75/0xc0
[<00000000946ee250>] do_syscall_64+0x3b/0x90
[<0000000000506b7f>] entry_SYSCALL_64_after_hwframe+0x63/0xcd

The root case here is: In function prepare_func_exit(), the callee is
not released in the abnormal scenario after "state->curframe--;". To
fix, move "state->curframe--;" to the very bottom of the function,
right when we free callee and reset frame[] pointer to NULL, as Andrii
suggested.

In addition, function __check_func_call() has a similar problem. In
the abnormal scenario before "state->curframe++;", the callee also
should be released by free_func_state().

Fixes: 69c087ba6225 ("bpf: Add bpf_for_each_map_elem() helper")
Fixes: fd978bf7fd31 ("bpf: Add reference tracking to verifier")
Signed-off-by: Wang Yufen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

kernel/bpf/verifier.c

···
 	/* Transfer references to the callee */
 	err = copy_reference_state(callee, caller);
 	if (err)
-		return err;
+		goto err_out;
 
 	err = set_callee_state_cb(env, caller, callee, *insn_idx);
 	if (err)
-		return err;
+		goto err_out;
 
 	clear_caller_saved_regs(env, caller->regs);
 
···
 		print_verifier_state(env, callee, true);
 	}
 	return 0;
+
+err_out:
+	free_func_state(callee);
+	state->frame[state->curframe + 1] = NULL;
+	return err;
 }
 
 int map_set_for_each_callback_args(struct bpf_verifier_env *env,
···
 		return -EINVAL;
 	}
 
-	state->curframe--;
-	caller = state->frame[state->curframe];
+	caller = state->frame[state->curframe - 1];
 	if (callee->in_callback_fn) {
 		/* enforce R0 return value range [0, 1]. */
 		struct tnum range = callee->callback_ret_range;
···
 	}
 	/* clear everything in the callee */
 	free_func_state(callee);
-	state->frame[state->curframe + 1] = NULL;
+	state->frame[state->curframe--] = NULL;
 	return 0;
 }