src/gpasswd.c: fix segfault in clean up callbacks

src/gpasswd.c: fix segfault in clean up callbacks

The gpasswd utility was segfaulting when cleanup functions were called
because these functions expect a pointer to `process_selinux` but was
being passed NULL. This caused a NULL pointer dereference.

This commits adds the pointer to `process_selinux` to clean up
functions making `gpasswd` consistent with other group utilities.

Reproduction steps:
$ useradd tuser
$ groupadd tuser
$ gpasswd -a tuser tgroup
Adding user tuser to group tgroup
Segmentation fault (core dumped)

Fixes: 4d431898bad8 (2025-10-07; "src/gpasswd.c: chroot or prefix SELinux file context")
Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>