mm/khugepaged: factor out common logic in [scan,alloc]_sleep_millisecs_store()

mm/khugepaged: factor out common logic in [scan,alloc]_sleep_millisecs_store()

Both scan_sleep_millisecs_store() and alloc_sleep_millisecs_store()
perform the same operations: parse the input value, update their
respective sleep interval, reset khugepaged_sleep_expire, and wake up the
khugepaged thread.

Factor out this duplicated logic into a helper function
__sleep_millisecs_store(), and simplify both store functions.

No functional change intended.

Link: https://lkml.kernel.org/r/20251021134431.26488-1-leon.hwang@linux.dev
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Nico Pache <npache@redhat.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>