efi_loader: create common function to free struct efi_disk_obj

Current error handling of creating raw disk/partition has
following issues.
 - duplicate free for EFI handle, EFI handle is already freed
   in efi_delete_handle()
 - missing free for struct efi_device_path and
   struct efi_simple_file_system_protocol in some error paths

To address those issues, this commit creates the common function
to free the struct efi_disk_obj resources and calls it in case
of error.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>