uapi: fix big endian definition of ipv6_rpl_sr_hdr

uapi: fix big endian definition of ipv6_rpl_sr_hdr

Following RFC 6554 [1], the current order of fields is wrong for big
endian definition. Indeed, here is how the header looks like:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Next Header  |  Hdr Ext Len  | Routing Type  | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CmprI | CmprE |  Pad  |               Reserved                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

This patch reorders fields so that big endian definition is now correct.

  [1] https://tools.ietf.org/html/rfc6554#section-3

Fixes: cfa933d938d8 ("include: uapi: linux: add rpl sr header definition")
Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>