efivarfs: Never return ENOENT from firmware

Files are created in efivarfs_create() before a corresponding variable
is created in the firmware. This leads to users being able to
read/write to the file without the variable existing in the
firmware. Reading a non-existent variable currently returns -ENOENT,
which is confusing because the file obviously *does* exist.

Convert EFI_NOT_FOUND into -EIO which is the closest thing to "error
while interacting with firmware", and should hopefully indicate to the
caller that the variable is in some uninitialised state.

Cc: Josh Boyer <jwboyer@redhat.com>
Acked-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Cc: Lee, Chun-Yi <jlee@suse.com>
Cc: Andy Whitcroft <apw@canonical.com>
Reported-by: Lingzhu Xiang <lxiang@redhat.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

drivers/firmware/efivars.c

@@ -674,7 +674,7 @@ static int efi_status_to_err(efi_status_t status)
 		err = -EACCES;
 		break;
 	case EFI_NOT_FOUND:
-		err = -ENOENT;
+		err = -EIO;
 		break;
 	default:
 		err = -EINVAL;