fs: move i_wb_list out from under inode_lock

Protect the inode writeback list with a new global lock
inode_wb_list_lock and use it to protect the list manipulations and
traversals. This lock replaces the inode_lock as the inodes on the
list can be validity checked while holding the inode->i_lock and
hence the inode_lock is no longer needed to protect the list.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 0ead399..3f5fee7 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -10,6 +10,7 @@
 struct backing_dev_info;
 
 extern spinlock_t inode_lock;
+extern spinlock_t inode_wb_list_lock;
 
 /*
  * fs/fs-writeback.c