mm/damon/reclaim: schedule 'damon_reclaim_timer' only after 'system_wq' is initialized

mm/damon/reclaim: schedule 'damon_reclaim_timer' only after 'system_wq' is initialized

Commit 6c8e96205347 ("mm/damon/reclaim: fix the timer always stays
active") made DAMON_RECLAIM's 'enabled' parameter store callback,
'enabled_store()', to schedule 'damon_reclaim_timer'.  The scheduling uses
'system_wq', which is initialized in 'workqueue_init_early()'.  As kernel
parameters parsing function ('parse_args()') is called before
'workqueue_init_early()', 'enabled_store()' can be executed before
'workqueue_init_early()' and end up accessing the uninitialized
'system_wq'.  As a result, the booting hang[1].  This commit fixes the
issue by checking if the initialization is done before scheduling the
timer.

[1] https://lkml.kernel.org/20220604192222.1488-1-sj@kernel.org/

Link: https://lkml.kernel.org/r/20220604195051.1589-1-sj@kernel.org
Fixes: 6c8e96205347 ("mm/damon/reclaim: fix the timer always stays active")
Signed-off-by: SeongJae Park <sj@kernel.org>
Reported-by: Greg White <gwhite@kupulau.com>
Cc: Hailong Tu <tuhailong@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>