vmxnet3: invoke vlib_buffer_enqueue_to_next() with the wrong buffers for chain buffers

vmxnet3: invoke vlib_buffer_enqueue_to_next() with the wrong buffers for chain buffers

If we invoke the subject API with 2 buffers, each with 2 chains, the content of
buffers[0] should be head of 1st buffer, and content of buffer[1] should be
head of 2nd buffer.

What the code did was to put
buffers[0] = head of 1st buffer
buffers[1] = next chain of 1st buffer
buffers[2] = head of 2nd buffer
buffers[3] = next chain of 2nd buffer

This is wrong and can cause crash.

The fix is to only increment bi when the entire packet is found which may consist
of multiple fragments.

Change-Id: If02cd5186a039d7a6c08a0959112840bdb242317
Signed-off-by: Steven <sluong@cisco.com>