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: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 26adea8..9dec97f 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -204,7 +204,7 @@
  * 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;