mm: page-writeback.c: local functions should not be exposed globally

The function global_dirtyable_memory is only referenced in this file and
should be marked static to prevent it from being exposed globally.

This quiets the sparse warning:

warning: symbol 'global_dirtyable_memory' was not declared. Should it be static?
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: NFengguang Wu <fengguang.wu@intel.com>

mm/page-writeback.c

@@ -204,7 +204,7 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
  * Returns the global number of pages potentially available for dirty
  * page cache.  This is the base value for the global dirty limits.
  */
-unsigned long global_dirtyable_memory(void)
+static unsigned long global_dirtyable_memory(void)
 {
 	unsigned long x;