New IOAM6 encap type for routes

New IOAM6 encap type for routes

This patch provides a new encap type for routes to insert an IOAM pre-allocated
trace:

$ ip -6 ro ad fc00::1/128 encap ioam6 trace prealloc type 0x800000 ns 1 size 12 dev eth0

where:
 - "trace" and "prealloc" may appear as useless but just anticipate for future
   implementations of other ioam option types.
 - "type" is a bitfield (=u32) defining the IOAM pre-allocated trace type (see
   the corresponding uapi).
 - "ns" is an IOAM namespace ID attached to the pre-allocated trace.
 - "size" is the trace pre-allocated size in bytes; must be a 4-octet multiple;
   limited size (see IOAM6_TRACE_DATA_SIZE_MAX).

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: David Ahern <dsahern@kernel.org>