coda: move backing-dev.h kernel include inside __KERNEL__

Otherwise we must export backing-dev.h as well, which doesn't make
any sense.
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

include/linux/coda_psdev.h

 #ifndef __CODA_PSDEV_H
 #define __CODA_PSDEV_H
 
-#include <linux/backing-dev.h>
 #include <linux/magic.h>
 
 #define CODA_PSDEV_MAJOR 67
 #define MAX_CODADEVS  5	   /* how many do we allow */
 
 #ifdef __KERNEL__
+#include <linux/backing-dev.h>
+
 struct kstatfs;
 
 /* communication pending/processing queues */