mm/writeback: make writeback_inodes_wb static

mm/writeback: make writeback_inodes_wb static

It's not used globally and could be static.

Signed-off-by: Wanpeng Li <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Fengguang Wu <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Yasuaki Ishimatsu <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Cc: Jiri Kosina <[email protected]>
Cc: Wanpeng Li <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>