This is a simple refactor, as there isn't any twisted logic in old
code. Here I just broke the code and introduced two helper functions,
reserve_avail_buf() and copy_mbuf_to_desc() to make the code more
readable.
Also, it saves nearly 1K bytes of binary code size.
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
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