tools/cgxset: Fix potential memory leak in converted_src_cgroup

tools/cgxset: Fix potential memory leak in converted_src_cgroup

There's a brief window where converted_src_cgroup is allocated but
hasn't been assigned to the *cgroup pointer.  If a failure occurs in
this window, then converted_src_cgroup must be freed before doing the
goto to the error path.

Also, simplify the error-handling logic in cgxset::main() by having all
errors goto "err".

Reviewed-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>