[S390] cio: fix wrong buffer access in cio_ignore_write

Writing only spaces to /proc/cio_ignore will cause a buffer overflow
since the size_t value i will not become negative and so buf[-1UL] is
accessed. Change the value of i to ssize_t.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>