When copying ipfs data from user space, don't just check that the payload
length is nonzero, but also that it does not exceed the size of the stack
buffer we're copying it into.
While we're at it, use a union to create a buffer of the exact size we
need instead of guessing that 2048 will be enough (and not too much).
Finally, check the size of the payload once it gets to where it's used.
MFC after: 3 days