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

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