efi_loader: Fix efi_exit gd clobbering

Commit f494950b (efi_loader: call __efi_exit_check in efi_exit) added a call
to __efi_exit_check inside efi_exit to account for the fact that we're exiting
the efi_exit function via a longjmp call.

However, __efi_exit_check also swizzles gd to the application gd while the
longjmp will put us back into EFI context, so we need the efi (u-boot) gd.

This patch fixes that up by explicitly setting gd back to efi_gd before
doing the longjmp. It also adds a few comments on why it does that.

Signed-off-by: Alexander Graf <agraf@suse.de>

lib/efi_loader/efi_boottime.c

@@ -798,8 +798,15 @@ static efi_status_t EFIAPI efi_exit(efi_handle_t image_handle,
 	EFI_ENTRY("%p, %ld, %ld, %p", image_handle, exit_status,
 		  exit_data_size, exit_data);
 
+	/* Make sure entry/exit counts for EFI world cross-overs match */
 	__efi_exit_check();
 
+	/*
+	 * But longjmp out with the U-Boot gd, not the application's, as
+	 * the other end is a setjmp call inside EFI context.
+	 */
+	efi_restore_gd();
+
 	loaded_image_info->exit_status = exit_status;
 	longjmp(&loaded_image_info->exit_jmp, 1);