loop: prevent information leak after failed read

If read was not fully successful we have to fail whole bio to prevent
information leak of old pages

##Testcase_begin
dd if=/dev/zero of=./file bs=1M count=1
losetup /dev/loop0 ./file -o 4096
truncate -s 0 ./file
# OOps loop offset is now beyond i_size, so read will silently fail.
# So bio's pages would not be cleared, may which result in information leak.
hexdump -C /dev/loop0
##testcase_end

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -372,7 +372,8 @@ do_lo_receive(struct loop_device *lo,
 
 	if (retval < 0)
 		return retval;
-
+	if (retval != bvec->bv_len)
+		return -EIO;
 	return 0;
 }