mm/damon/stat: fix memory leak on damon_start() failure in damon_stat_start()

mm/damon/stat: fix memory leak on damon_start() failure in damon_stat_start()

Destroy the DAMON context and reset the global pointer when damon_start()
fails.  Otherwise, the context allocated by damon_stat_build_ctx() is
leaked, and the stale damon_stat_context pointer will be overwritten on
the next enable attempt, making the old allocation permanently
unreachable.

Link: https://lore.kernel.org/20260331101553.88422-1-liu.yun@linux.dev
Fixes: 369c415e6073 ("mm/damon: introduce DAMON_STAT module")
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 6.17.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>