bpf: Fix memory leaks in __check_func_call

In the Linux kernel, the following vulnerability has been resolved:

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().