fs: block_page_mkwrite should wait for writeback to finish

For filesystems such as nilfs2 and xfs that use block_page_mkwrite, modify that
function to wait for pending writeback before allowing the page to become
writable.  This is needed to stabilize pages during writeback for those two
filesystems.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/buffer.c

@@ -2382,6 +2382,7 @@ int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
 		ret = -EAGAIN;
 		goto out_unlock;
 	}
+	wait_on_page_writeback(page);
 	return 0;
 out_unlock:
 	unlock_page(page);