fs: kill BDI_CAP_SWAP_BACKED

This bdi flag isn't too useful - we can determine that a vma is backed by
either swap or shmem trivially in the caller.

This also allows removing the backing_dev_info instaces for swap and shmem
in favor of noop_backing_dev_info.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

mm/swap_state.c

@@ -32,17 +32,12 @@ static const struct address_space_operations swap_aops = {
 #endif
 };
 
-static struct backing_dev_info swap_backing_dev_info = {
-	.name		= "swap",
-	.capabilities	= BDI_CAP_NO_ACCT_AND_WRITEBACK | BDI_CAP_SWAP_BACKED,
-};
-
 struct address_space swapper_spaces[MAX_SWAPFILES] = {
 	[0 ... MAX_SWAPFILES - 1] = {
 		.page_tree	= RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
 		.i_mmap_writable = ATOMIC_INIT(0),
 		.a_ops		= &swap_aops,
-		.backing_dev_info = &swap_backing_dev_info,
+		.backing_dev_info = &noop_backing_dev_info,
 	}
 };