sparc64: viohs: Remove VLA usage

In the quest to remove all stack VLA usage from the kernel[1], this
allocates a fixed size array for the maximum number of cookies and
adds a runtime sanity check.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1
RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>