mm: fix unused variable kernel warning when SYSCTL=n
When CONFIG_SYSCTL=n the variable dirty_bytes_min which is just used
as a minimum to a proc handler is not used. So just move this under
the ifdef for CONFIG_SYSCTL.
Fixes: aa779e510219 ("mm: move page-writeback sysctls to their own file") Reported-by: kernel test robot <lkp@intel.com> Acked-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>