x86: split ret_from_fork

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 8ec3a1a..d93eb9d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -97,6 +97,7 @@
 	select KTIME_SCALAR if X86_32
 	select GENERIC_STRNCPY_FROM_USER
 	select GENERIC_STRNLEN_USER
+	select GENERIC_KERNEL_THREAD
 
 config INSTRUCTION_DECODER
 	def_bool (KPROBES || PERF_EVENTS || UPROBES)