mm/damon/core: make damon_start() waits until kdamond_fn() starts

The cleanup tasks of kdamond threads including reset of corresponding
DAMON context's ->kdamond field and decrease of global nr_running_ctxs
counter is supposed to be executed by kdamond_fn().  However, commit
0f91d13366a4 ("mm/damon: simplify stop mechanism") made neither
damon_start() nor damon_stop() ensure the corresponding kdamond has
started the execution of kdamond_fn().

As a result, the cleanup can be skipped if damon_stop() is called fast
enough after the previous damon_start().  Especially the skipped reset
of ->kdamond could cause a use-after-free.

Fix it by waiting for start of kdamond_fn() execution from
damon_start().

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 0f91d13366a4 ("mm/damon: simplify stop mechanism")
Signed-off-by: SeongJae Park <[email protected]>
Reported-by: Jakub Acs <[email protected]>
Cc: Changbin Du <[email protected]>
Cc: Jakub Acs <[email protected]>
Cc: <[email protected]> # 5.15.x
Signed-off-by: Andrew Morton <[email protected]>

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