mm/damon/vaddr: add a comment for 'default' case in damon_va_apply_scheme()

mainline inclusion
from mainline-v6.2-rc6
commit 5934ec13
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6MRUG

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5934ec1362b235c4341807c28f79b6a596ce1b40

--------------------------------

The switch case 'DAMOS_STAT' and switch case 'default' have same return
value in damon_va_apply_scheme(), and the 'default' case is for DAMOS
actions that not supported by 'vaddr'.  It might make sense to add a
comment here.

[akpm@linux-foundation.org: fx comment grammar]
Link: https://lkml.kernel.org/r/1662606797-23534-1-git-send-email-kaixuxia@tencent.com


Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ze Zuo <zuoze1@huawei.com>