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: aa779e51 ("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>