s390: use r13 in cleanup_sie as temp register

Instead of thrashing r11 which is normally our pointer to struct
pt_regs on the stack, use r13 as temporary register in the BR_EX
macro. r13 is already used in cleanup_sie, so no need to thrash
another register.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>