Eliminate deadlock in forked child due to delayed resetting mutex lock

For some program implementation, the pattern like below, calling 
pthread_atfork to register atfork interfaces.

    pthread_atfork(&atfork_prepare, &atfork_parent, &atfork_child);

When the program is expected to reopen the shared library's handle 
inherited from parent in child process. Maybe, dlclose is called in 
atfork_child to release the shared library handle before reopen it. 
Then, dlclose will indrectly call _cxa_finalize and finaly call 
__unregister_atfork when dso is not NULL.

    atfork_child() -> dlclose() -> __on_dlclose() 
    	-> __cxa_finalize() -> __unregister_atfork(dso)

In __unregister_atfork, firstly, it try to hold the g_atfork_list_mutex 
lock to operate the g_atfork_list. Due to the registered atfork_child is
executed before resetting g_atfork_list_mutex lock in child, the child 
process will be blocked here because of deadlock.

Test: bionic-unit-tests32 --gtest_filter=pthread.pthread_atfork_child_with_dlclose 
without the fixing, the test will be timeout.

Change-Id: I35d3001682c836e0955d6d681bc5f9297fad0c7b
Signed-off-by: Mingwei Shi <mingwei.shi@intel.com>
Signed-off-by: Qiming Shi <qiming.shi@intel.com>
Signed-off-by: Chao Xie <chao.xie@intel.com>