IDMAP: Fix computing max id for slice range
Max value of id mapping range was 1 unit too high.
Resolves:
https://fedorahosted.org/sssd/ticket/2922
Reviewed-by: Sumit Bose <sbose@redhat.com>
(cherry picked from commit 7db89d44b5582a0cb0a61a7aa42a2fac7ca9408f)