src: fix routing header support

src: fix routing header support

We can't use nft_exthdr_op to encode routing header, it breaks
ipv6 extension header support.

When encountering RT header, userspace did now set a new ipv6 exthdr mode,
but old kernel doesn't know about this, so this failed with -EOPNOTSUPP.

Revert that part and use NFT_EXTHDR_OP_IPV6.
When decoding a routing extension header, try the various route
types until we find a match.

Note this patch isn't complete:

'srh tag 127' creates following expressions:
  [ exthdr load 2b @ 43 + 6 => reg 1 ]
  [ cmp eq reg 1 0x00007f00 ]

It should instead insert a dependency test ("rt type 4"):
  [ exthdr load 1b @ 43 + 2 => reg 1 ]
  [ cmp eq reg 1 0x00000004 ]
  [ exthdr load 2b @ 43 + 6 => reg 1 ]
  [ cmp eq reg 1 0x00007e00 ]

nft should then use this to infer the routing header type.

While add it, document the srh option.

Fixes: 1400288f6d39d ("src: handle rt0 and rt2 properly")
Reported-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Ahmed Abdelsalam <amsalam20@gmail.com>