[S390] Initialize __LC_THREAD_INFO early.

"lockdep: Fix backtraces" reveales a bug in early setup code: when
lockdep tries to save a stack backtrace before setup_arch has been
called the lowcore pointer for the current thread info pointer isn't
initialized yet.
However our save stack backtrace code relies on it. If the pointer
isn't initialized the saved backtrace will have zero entries.
lockdep however relies (correctly) on the fact that that cannot
happen.
A write access to some random memory region is the result.

Fix this by initializing the thread info pointer early.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/head31.S

@@ -24,6 +24,7 @@ startup_continue:
 # Setup stack
 #
 	l	%r15,.Linittu-.LPG1(%r13)
+	st	%r15,__LC_THREAD_INFO	# cache thread info in lowcore
 	mvc	__LC_CURRENT(4),__TI_task(%r15)
 	ahi	%r15,1<<(PAGE_SHIFT+THREAD_ORDER) # init_task_union+THREAD_SIZE
 	st	%r15,__LC_KERNEL_STACK	# set end of kernel stack

arch/s390/kernel/head64.S

@@ -92,6 +92,7 @@ startup_continue:
 # Setup stack
 #
 	larl	%r15,init_thread_union
+	stg	%r15,__LC_THREAD_INFO	# cache thread info in lowcore
 	lg	%r14,__TI_task(%r15)	# cache current in lowcore
 	stg	%r14,__LC_CURRENT
 	aghi	%r15,1<<(PAGE_SHIFT+THREAD_ORDER) # init_task_union + THREAD_SIZE