common/mutex_debug: disable lockdep for recursive_mutex

common/mutex_debug: disable lockdep for recursive_mutex

without this change, when using recursive_mutex on Debug build, we will
have

 1: (ceph::mutex_debug_detail::mutex_debugging_base::_will_lock(bool)+0x4a)
[0x7fb751e36534]
 2: (ceph::mutex_debug_detail::mutex_debug_impl<true>::lock(bool)+0x3b)
[0x7fb751ca33b5]
 3: (std::lock_guard<ceph::mutex_debug_detail::mutex_debug_impl<true>
>::lock_guard(ceph::mutex_debug_detail::mutex_debug_impl<true>&)+0x2f)
[0x7fb751ca2e79]
 4: (ConfigProxy::get_val(std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > const&, char**, int)
const+0x41) [0x7fb751ce65fb]
 5: (TracepointProvider::verify_config(ConfigProxy const&)+0xbf)
[0x7fb751ce63f3]
 6: (TracepointProvider::handle_conf_change(ConfigProxy const&,
std::set<std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> >, std::less<std::__cx
x11::basic_string<char, std::char_traits<char>, std::allocator<char> >
>, std::allocator<std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > > >
const&)+0x93) [0x7fb751ce62eb]
 7: (void ObserverMgr<ceph::md_config_obs_impl<ConfigProxy>
>::call_all_observers<ConfigProxy>(ConfigProxy const&)+0x1ab)
[0x7fb751d4f1e1]
 8: (ConfigProxy::call_all_observers()+0x50) [0x7fb751d4c570]
 9: (CephContext::start_service_thread()+0x121) [0x7fb751d47eff]

because we have following sequence:

1. acquire ConfigProxy::lock
2. acquire TracepointProvider::m_lock
3. acquire ConfigProxy::lock

which annoys lockdep, because it's a cyclic graph.

after this change,

* we will not ignore lockdep when locking/unlocking
  recursive_mutex. this basically disables lockdep checks for
  mutex_recursive_debug, but it still has nlock checks.
* also, enable_lockdep() is extracted. as we are repeating it
  for multiple times in this header file.

see 41522699, and see http://tracker.ceph.com/issues/12614

Signed-off-by: Kefu Chai <kchai@redhat.com>