efivarfs: efivarfs_fill_super() fix inode reference counts

When d_make_root() fails it will automatically drop the reference
on the root inode.  We should not be doing so as well.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Matthew Garrett <mjg@redhat.com>
Acked-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>