ifp: use realloc in ifp_list_ctx_remaining_capacity()
    
    ifp_list_ctx_remaining_capacity() might be called multiple times if
    results from multiple domains are added to the result list.
    
    The current use of talloc_zero_array() which was introduced with commit
    b0b9222 will override results which are already in the list. This causes
    a regression since it worked before.
    
    This patch replaces it with talloc_realloc().
    
    Resolves https://pagure.io/SSSD/sssd/issue/3608
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>