SafeSetID: fix UID printed instead of GID

pr_warn message clearly says that GID should be printed,
but we have UID there. Let's fix that.

Found accidentally during the work on isolated user namespaces.

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
[PM: fix spelling errors in description, subject tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>