Re-share zfsdev_getminor and zfs_onexit_fd_hold

By adding a zfs_file_private accessor to the common
interfaces and some extensions to FreeBSD platform
code it is now possible to share the implementations
for the aforementioned functions.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10073

module/os/linux/zfs/zfs_file_os.c

@@ -373,6 +373,19 @@ zfs_file_off(zfs_file_t *fp)
 	return (fp->f_pos);
 }
 
+/*
+ * Request file pointer private data
+ *
+ * fp - pointer to file
+ *
+ * Returns pointer to file private data.
+ */
+void *
+zfs_file_private(zfs_file_t *fp)
+{
+	return (fp->private_data);
+}
+
 /*
  * unlink file
  *