mm/damon/core: skip apply schemes if empty

mm/damon/core: skip apply schemes if empty

Sometimes there is no scheme in damon's context, for example just use damo
record to monitor workload's data access pattern.

If current damon context doesn't have any scheme in the list, kdamond has
no need to iterate over list of all targets and regions but do nothing.

So, skip apply schemes when ctx->schemes is empty.

Link: https://lkml.kernel.org/r/20230116062347.1148553-1-huaisheng.ye@intel.com
Signed-off-by: Huaisheng Ye <huaisheng.ye@intel.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>