x86: use _do_fork() in compat_sys_x86_clone()

It is trivial to directly call _do_fork() instead of the sys_clone()
syscall in compat_sys_x86_clone().

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: x86@kernel.org
Reviewed-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NDominik Brodowski <linux@dominikbrodowski.net>

arch/x86/ia32/sys_ia32.c

@@ -41,6 +41,7 @@
 #include <linux/highuid.h>
 #include <linux/sysctl.h>
 #include <linux/slab.h>
+#include <linux/sched/task.h>
 #include <asm/mman.h>
 #include <asm/types.h>
 #include <linux/uaccess.h>
@@ -241,6 +242,6 @@ COMPAT_SYSCALL_DEFINE5(x86_clone, unsigned long, clone_flags,
 		       unsigned long, newsp, int __user *, parent_tidptr,
 		       unsigned long, tls_val, int __user *, child_tidptr)
 {
-	return sys_clone(clone_flags, newsp, parent_tidptr, child_tidptr,
+	return _do_fork(clone_flags, newsp, 0, parent_tidptr, child_tidptr,
 			tls_val);
 }