ip: add support for more MPLS labels

ip: add support for more MPLS labels

Kernel now supports up to 30 labels but not defined as part of the uapi.
iproute2 handles up to 8 labels but in a non-consistent way. Update ip
to handle more labels, but in a more programmatic way.

For the MPLS address family, the data field in inet_prefix is used for
labels.  Increase that field to 64 u32's -- 64 as nothing more than a
convenient power of 2 number.

Update mpls_pton to take the length of the address field, convert that
length to number of labels and add better error handling to the parsing
of the user supplied string.

Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>