[GFS2] Add a comment in ops_export.c

Ass a comment explaining the slightly odd construct used to
pass error values back.

Cc: Jan Engelhardt <jengelh@linux01.gwdg.de>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/ops_export.c

@@ -182,6 +182,10 @@ static struct dentry *gfs2_get_parent(struct dentry *child)
 
 	if (!inode)
 		return ERR_PTR(-ENOENT);
+	/*
+	 * In case of an error, @inode carries the error value, and we
+	 * have to return that as a(n invalid) pointer to dentry.
+	 */
 	if (IS_ERR(inode))
 		return ERR_PTR(PTR_ERR(inode));