bpf: cpumap: Fix memory leak in cpu_map_update_elem

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

bpf: cpumap: Fix memory leak in cpu_map_update_elem

Syzkaller reported a memory leak as follows:

BUG: memory leak
unreferenced object 0xff110001198ef748 (size 192):
comm "syz-executor.3", pid 17672, jiffies 4298118891 (age 9.906s)
hex dump (first 32 bytes):
00 00 00 00 4a 19 00 00 80 ad e3 e4 fe ff c0 00 ....J...........
00 b2 d3 0c 01 00 11 ff 28 f5 8e 19 01 00 11 ff ........(.......
backtrace:
[<ffffffffadd28087>] __cpu_map_entry_alloc+0xf7/0xb00
[<ffffffffadd28d8e>] cpu_map_update_elem+0x2fe/0x3d0
[<ffffffffadc6d0fd>] bpf_map_update_value.isra.0+0x2bd/0x520
[<ffffffffadc7349b>] map_update_elem+0x4cb/0x720
[<ffffffffadc7d983>] __se_sys_bpf+0x8c3/0xb90
[<ffffffffb029cc80>] do_syscall_64+0x30/0x40
[<ffffffffb0400099>] entry_SYSCALL_64_after_hwframe+0x61/0xc6

BUG: memory leak
unreferenced object 0xff110001198ef528 (size 192):
comm "syz-executor.3", pid 17672, jiffies 4298118891 (age 9.906s)
hex dump (first 32 bytes):
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 00 ................
backtrace:
[<ffffffffadd281f0>] __cpu_map_entry_alloc+0x260/0xb00
[<ffffffffadd28d8e>] cpu_map_update_elem+0x2fe/0x3d0
[<ffffffffadc6d0fd>] bpf_map_update_value.isra.0+0x2bd/0x520
[<ffffffffadc7349b>] map_update_elem+0x4cb/0x720
[<ffffffffadc7d983>] __se_sys_bpf+0x8c3/0xb90
[<ffffffffb029cc80>] do_syscall_64+0x30/0x40
[<ffffffffb0400099>] entry_SYSCALL_64_after_hwframe+0x61/0xc6

BUG: memory leak
unreferenced object 0xff1100010fd93d68 (size 8):
comm "syz-executor.3", pid 17672, jiffies 4298118891 (age 9.906s)
hex dump (first 8 bytes):
00 00 00 00 00 00 00 00 ........
backtrace:
[<ffffffffade5db3e>] kvmalloc_node+0x11e/0x170
[<ffffffffadd28280>] __cpu_map_entry_alloc+0x2f0/0xb00
[<ffffffffadd28d8e>] cpu_map_update_elem+0x2fe/0x3d0
[<ffffffffadc6d0fd>] bpf_map_update_value.isra.0+0x2bd/0x520
[<ffffffffadc7349b>] map_update_elem+0x4cb/0x720
[<ffffffffadc7d983>] __se_sys_bpf+0x8c3/0xb90
[<ffffffffb029cc80>] do_syscall_64+0x30/0x40
[<ffffffffb0400099>] entry_SYSCALL_64_after_hwframe+0x61/0xc6

In the cpu_map_update_elem flow, when kthread_stop is called before
calling the threadfn of rcpu->kthread, since the KTHREAD_SHOULD_STOP bit
of kthread has been set by kthread_stop, the threadfn of rcpu->kthread
will never be executed, and rcpu->refcnt will never be 0, which will
lead to the allocated rcpu, rcpu->queue and rcpu->queue->queue cannot be
released.

Calling kthread_stop before executing kthread's threadfn will return
-EINTR. We can complete the release of memory resources in this state.