fs: export empty_aops

With the ->sync_page() hook gone, we have a few users that
add their own static address_space_operations without any
functions defined.

fs/inode.c already has an empty_aops that it uses for init
purposes. Lets export that and use it in the places where
an otherwise empty aops was defined.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

fs/nilfs2/page.c

@@ -495,8 +495,6 @@ unsigned nilfs_page_count_clean_buffers(struct page *page,
 void nilfs_mapping_init(struct address_space *mapping,
 			struct backing_dev_info *bdi)
 {
-	static const struct address_space_operations empty_aops;
-
 	mapping->host = NULL;
 	mapping->flags = 0;
 	mapping_set_gfp_mask(mapping, GFP_NOFS);