Revert "rcu: do not create thread in pthread_atfork callback"

This reverts commit a59629fcc6.
This is not needed anymore because the IOThread mutex is not
"magic" anymore (need not kick the CPU thread)and also because
fork callbacks are only enabled at the very beginning of
QEMU's execution.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

os-posix.c

@@ -34,7 +34,6 @@
 #include "sysemu/sysemu.h"
 #include "net/slirp.h"
 #include "qemu-options.h"
-#include "qemu/rcu.h"
 #include "qemu/error-report.h"
 #include "qemu/log.h"
 #include "qemu/cutils.h"
@@ -249,7 +248,6 @@ void os_daemonize(void)
         signal(SIGTSTP, SIG_IGN);
         signal(SIGTTOU, SIG_IGN);
         signal(SIGTTIN, SIG_IGN);
-        rcu_after_fork();
     }
 }