common/util: silent "warning: ‘cgroup_limit’ may be used uninitialized"

common/util: silent "warning: ‘cgroup_limit’ may be used uninitialized"

Full output:
/home/if/ceph.3/src/common/util.cc: In function ‘void collect_sys_info(std::map<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >*, ceph::common::CephContext*)’:
/home/if/ceph.3/src/common/util.cc:360:51: warning: ‘cgroup_limit’ may be used uninitialized [-Wmaybe-uninitialized]
  360 |   if (get_cgroup_memory_limit(&cgroup_limit) == 0 &&
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
  361 |       cgroup_limit > 0) {
      |       ~~~~~~~~~~~~~~~~
/home/if/ceph.3/src/common/util.cc:359:12: note: ‘cgroup_limit’ was declared here
  359 |   uint64_t cgroup_limit;
      |            ^~~~~~~~~~~~

Signed-off-by: Igor Fedotov <igor.fedotov@croit.io>