src/socket_wrapper.c: fix mutex fork handling

src/socket_wrapper.c: fix mutex fork handling

We need to use pthread_mutex_init in the child handler...
See https://sourceware.org/bugzilla/show_bug.cgi?id=2745

Valgrind tools like helgrind and drd don't understand this
(at least in 3.15.0), they require a pthread_mutex_unlock()
in the child in order work.

Pair-Programmed-With: Andreas Schneider <asn@samba.org>

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Andreas Schneider <asn@samba.org>