Eliminate deadlock in forked child due to delayed resetting mutex lock

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>