[XFS] Fix a couple of issues on the error path when dealing with external

devices (log/realtime).  path_init was missing the LOOKUP_POSITIVE
flag, so it would fail to tell us if the file doesn't exist, there
was a spot where we were returning the wrong signedness for the code,
and when mount is failing, we can call into xfs_blkdev_put with a
NULL pointer depending on which devices were initialised and which
weren't.

SGI Modid: 2.5.x-xfs:slinx:131469a

fs/xfs/linux/xfs_super.c

@@ -477,9 +477,8 @@ xfs_blkdev_get(
 
 	error = path_lookup(name, LOOKUP_FOLLOW, &nd);
 	if (error) {
-		printk("XFS: Invalid device [%s], error=%d\n",
-				name, error);
-		return error;
+		printk("XFS: Invalid device [%s], error=%d\n", name, error);
+		return -error;
 	}
 
 	/* I think we actually want bd_acquire here..  --hch */
@@ -498,6 +497,7 @@ void
 xfs_blkdev_put(
 	struct block_device	*bdev)
 {
+	if (bdev)
 		blkdev_put(bdev, BDEV_FS);
 }