include: uapi: linux: add rpl sr header definition

This patch adds a uapi header for rpl struct definition. The segments
data can be accessed over rpl_segaddr or rpl_segdata macros. In case of
compri and compre is zero the segment data is not compressed and can be
accessed by rpl_segaddr. In the other case the compressed data can be
accessed by rpl_segdata and interpreted as byte array.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>