s390/spinlock: fix system hang with spin_retry <= 0

On LPAR, when spin_retry is set to <= 0, arch_spin_lock_wait() and
arch_spin_lock_wait_flags() may end up in a while(1) loop w/o doing
any compare and swap operation. To fix this, use do/while instead of
for loop.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>