kernfs: implement kernfs_syscall_ops->remount_fs() and ->show_options()

Add two super_block related syscall callbacks ->remount_fs() and
->show_options() to kernfs_syscall_ops.  These simply forward the
matching super_operations.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>