mm: export dirty_writeback_interval

Export 'dirty_writeback_interval' to make it visible to
file-systems. We are going to push superblock management down to
file-systems and get rid of the 'sync_supers' kernel thread completly.
Signed-off-by: NArtem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

mm/page-writeback.c

@@ -95,6 +95,8 @@ unsigned long vm_dirty_bytes;
  */
 unsigned int dirty_writeback_interval = 5 * 100; /* centiseconds */
 
+EXPORT_SYMBOL_GPL(dirty_writeback_interval);
+
 /*
  * The longest time for which data is allowed to remain dirty
  */