block: move bdi/address_space unplug functions to backing-dev.h

There's nothing block related about them, the backing device
is used by things like NFS etc as well. This gets rid of the
need to protect such calls by CONFIG_BLOCK.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fs/aio.c b/fs/aio.c
index cf0bef4..c30dfc0 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -15,6 +15,7 @@
 #include <linux/aio_abi.h>
 #include <linux/module.h>
 #include <linux/syscalls.h>
+#include <linux/backing-dev.h>
 #include <linux/uio.h>
 
 #define DEBUG 0