mm: move oom_kill sysctls to their own file

kernel/sysctl.c is a kitchen sink where everyone leaves their dirty
dishes, this makes it very difficult to maintain.

To help with this maintenance let's start by moving sysctls to places
where they actually belong.  The proc sysctl maintainers do not want to
know what sysctl knobs you wish to add for your own piece of code, we just
care about the core logic.

So move the oom_kill sysctls to their own file, mm/oom_kill.c

[sfr@canb.auug.org.au: null-terminate the array]
  Link: https://lkml.kernel.org/r/20220216193202.28838626@canb.auug.org.au

Link: https://lkml.kernel.org/r/20220215093203.31032-1-sujiaxun@uniontech.com


Signed-off-by: sujiaxun <sujiaxun@uniontech.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Kees Cook <keescook@chromium.org>
Cc: Iurii Zaikin <yzaikin@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>