efivarfs: Allow unloading when build as module

There is no need to keep the module loaded when it serves no function in
case the EFI runtime services are disabled. Return an error in this case
so loading the module will fail.

Also supply a module_exit function to allow unloading the module.

Last, but not least, set the owner of the file_system_type struct.

Cc: Jeremy Kerr <jk@ozlabs.org>
Cc: Matthew Garrett <matthew.garrett@nebula.com>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>