staging: lustre: fix error deref in ll_splice_alias().

d_splice_alias() can return an ERR_PTR().
If it does while debugging is enabled, the following
CDEBUG() will dereference that error and crash.

So add appropriate checking, and provide a separate
debug message for the error case.

Reported-and-tested-by: James Simmons <jsimmons@infradead.org>
Fixes: e9d4f0b9f559 ("staging: lustre: llite: use d_splice_alias for directories.")
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/llite/namei.c

@@ -442,11 +442,16 @@ struct dentry *ll_splice_alias(struct inode *inode, struct dentry *de)
 	} else {
 		struct dentry *new = d_splice_alias(inode, de);
 
+		if (IS_ERR(new))
+			CDEBUG(D_DENTRY,
+			       "splice inode %p as %pd gives error %lu\n",
+			       inode, de, PTR_ERR(new));
 		if (new)
 			de = new;
 	}
-	CDEBUG(D_DENTRY, "Add dentry %p inode %p refc %d flags %#x\n",
-	       de, d_inode(de), d_count(de), de->d_flags);
+	if (!IS_ERR(de))
+		CDEBUG(D_DENTRY, "Add dentry %p inode %p refc %d flags %#x\n",
+		       de, d_inode(de), d_count(de), de->d_flags);
 	return de;
 }