If "hlen" less than "cutlen" then when we call upcall_msg_size()
the "hlen - cutlen" parameter will be a very high positive
number.
Later in the function we use "skb->len - cutlen" but this change
addresses that potential underflow since skb->len is always going to
be greater than or equal to hlen.
Fixes: f2a4d086ed4c ("openvswitch: Add packet truncation support.")
Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
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