struct dst_entry::dev is used most often. Move it so it can be
accessed without imm8 offset on x86_64.

	add/remove: 0/0 grow/shrink: 9/239 up/down: 52/-413 (-361)
	function                                     old     new   delta
	dst_rcu_free                                 126     138     +12
	fnhe_flush_routes                            211     219      +8
	rt_set_nexthop                               747     754      +7
	rt_cache_route                                85      91      +6
	rt6_release                                  209     215      +6
	dst_release                                  107     111      +4
	dst_destroy_rcu                               29      33      +4
	dn_dst_check_expire                          329     333      +4
	dn_insert_route                              484     485      +1
	xfrm_resolve_and_create_bundle              2991    2990      -1
					...
	ip_route_me_harder                          1163    1157      -6
	__ip_append_data.isra                       2730    2724      -6
	ip6_forward                                 3052    3045      -7
	callforward_do_filter                        659     651      -8
	dst_gc_task                                  571     549     -22
Signed-off-by: NAlexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>