USB: wusbcore: crypto: Remove VLA usage

In preparation to enabling -Wvla, remove VLA and replace it
with dynamic memory allocation instead.

The use of stack Variable Length Arrays needs to be avoided, as they
can be a vector for stack exhaustion, which can be both a runtime bug
or a security flaw. Also, in general, as code evolves it is easy to
lose track of how big a VLA can get. Thus, we can end up having runtime
failures that are hard to debug.

Also, fixed as part of the directive to remove all VLAs from
the kernel: https://lkml.org/lkml/2018/3/7/621

Notice that in this particular case, an alternative to kzalloc is kcalloc,
in which case the code would look as follows instead:

iv = kcalloc(crypto_skcipher_ivsize(tfm_cbc), sizeof(*iv), GFP_KERNEL);

but if the data type of _iv_ never changes, or the type size is always one
byte, kzalloc is good enough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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