security/sssd2: Fix two bugs in the find_uid patch

The UID check was added in commit 9a40ff81f4ea ("security/sssd2:
Reimplement UID-finding using sysctl") and is supposed to exclude kernel
processes, retaining the behaviour of commit ee81c8e455ef
("security/sssd2: Add patch to enumerate UIDs").

However, it mistakenly skipped over processes with UID 0, which could
cause the lookup to fail. Fix the typo.

Also return success if the caller asked us to populate a table and no
entries were added. This matches the previous behaviour.

PR: 284089
Approved by: jhixson (maintainer timeout)
Reviewed by: arrowd
Sponsored by: Klara, Inc.