[S390] Remove initial kernel stack backchain initialization.

Early init code clears the backchain of the initial kernel stack frame.
This is not necessary since it is pre initialized with zeros. Plus it
was broken on 64 bit since it cleared only four of eight bytes.
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/head31.S

@@ -29,7 +29,6 @@ startup_continue:
 	ahi	%r15,1<<(PAGE_SHIFT+THREAD_ORDER) # init_task_union+THREAD_SIZE
 	st	%r15,__LC_KERNEL_STACK	# set end of kernel stack
 	ahi	%r15,-96
-	xc	__SF_BACKCHAIN(4,%r15),__SF_BACKCHAIN(%r15) # clear backchain
 #
 # Save ipl parameters, clear bss memory, initialize storage key for kernel pages,
 # and create a kernel NSS if the SAVESYS= parm is defined

arch/s390/kernel/head64.S

@@ -96,7 +96,6 @@ startup_continue:
 	aghi	%r15,1<<(PAGE_SHIFT+THREAD_ORDER) # init_task_union + THREAD_SIZE
 	stg	%r15,__LC_KERNEL_STACK	# set end of kernel stack
 	aghi	%r15,-160
-	xc	__SF_BACKCHAIN(4,%r15),__SF_BACKCHAIN(%r15) # clear backchain
 #
 # Save ipl parameters, clear bss memory, initialize storage key for kernel pages,
 # and create a kernel NSS if the SAVESYS= parm is defined