fs/proc: Introduce list_for_each_table_entry for proc sysctl

Use the list_for_each_table_entry macro to optimize the scenario
of traverse ctl_table. This make the code neater and easier to
understand.

Suggested-by: Davidlohr <Bueso&lt;dave@stgolabs.net>
Signed-off-by: Meng Tang <tangmeng@uniontech.com>
[updated the sysctl_check_table() hunk due to some changes upstream]
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>