Fix typo we've had for a long time in set_re_uid() in the USE_SETRESUID case.

Fix typo we've had for a long time in set_re_uid() in the USE_SETRESUID case.

We only set the real euid, not the effective one. This is not
a security issue as this is *only* used in the quota code, and
only between code that brackets it with save_re_uid()/restore_re_uid(),
Also this is not used on most platforms (we use USE_SETREUID by
preference) but it's better to have this right. Bug to follow to get this
fixed in 3.6.next and 3.5.next.