mm/damon/core: clear walk_control on inactive context in damos_walk()

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

mm/damon/core: clear walk_control on inactive context in damos_walk()

damos_walk() sets ctx->walk_control to the caller-provided control

structure before checking whether the context is running. If the context

is inactive (damon_is_running() returns false), the function returns

-EINVAL without clearing ctx->walk_control. This leaves a dangling

pointer to a stack-allocated structure that will be freed when the caller

returns.

This is structurally identical to the bug fixed in commit f9132fbc2e83

("mm/damon/core: remove call_control in inactive contexts") for

damon_call(), which had the same pattern of linking a control object and

returning an error without unlinking it.

The dangling walk_control pointer can cause:

1. Use-after-free if the context is later started and kdamond

dereferences ctx->walk_control (e.g., in damos_walk_cancel()

which writes to control->canceled and calls complete())

2. Permanent -EBUSY from subsequent damos_walk() calls, since the

stale pointer is non-NULL

Nonetheless, the real user impact is quite restrictive. The

use-after-free is impossible because there is no damos_walk() callers who

starts the context later. The permanent -EBUSY can actually confuse

users, as DAMON is not running. But the symptom is kept only while the

context is turned off. Turning it on again will make DAMON internally

uses a newly generated damon_ctx object that doesn't have the invalid

damos_walk_control pointer, so everything will work fine again.

Fix this by clearing ctx->walk_control under walk_control_lock before

returning -EINVAL, mirroring the fix pattern from f9132fbc2e83.