lib/fs: Fix single return points for get_cgroup2_*

lib/fs: Fix single return points for get_cgroup2_*

Functions get_cgroup2_id() and get_cgroup2_path() may call close() with
a negative argument.
Avoid that making the calls conditional on the file descriptors.

get_cgroup2_path() may also return NULL leaking a file descriptor.
Ensure this does not happen using a single return point.

Fixes: d5e6ee0dac64 ("ss: introduce cgroup2 cache and helper functions")
Fixes: 8f1cd119b377 ("lib: fix checking of returned file handle size for cgroup")
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>