s2io bogus memset

memset() after kmalloc() on size * 8 would better be on size * 8, not
just size; fixed by switching to kcalloc() - it's more idiomatic anyway.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jeff Garzik <jeff@garzik.org>