nfs idmap: fix infinite loop
Resolves:
https://fedorahosted.org/sssd/ticket/2909
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
Reviewed-by: Noam Meltzer <tsnoam@gmail.com>
(cherry picked from commit 2a256e4e4b64891fe846e933589506daa68aa13e)
(cherry picked from commit 93e404ea1b3bda5961fb61e6d22d081adfed889a)