Fix warnings with !CONFIG_BLOCK

Hide everything in blkdev.h with CONFIG_BLOCK isn't set, and fixup
the (few) files that fail to build because they were relying on blkdev.h
pulling in extra includes for them.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index a4b142a..8d23b0b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -14,6 +14,7 @@
  */
 
 #include <linux/kernel.h>
+#include <linux/module.h>
 #include <linux/spinlock.h>
 #include <linux/sched.h>
 #include <linux/fs.h>