If get_rt_realms() fails, try to get a possible raw u32 realms
value for the u32 RTA_FLOW/FRA_FLOW attribute, as it might be
useful to directly configure the hex value itself. And only if
that fails, then bail out.

The source realm is provided in the upper u16 (mask: 0xffff0000)
and the destination realm through the lower u16 part (mask:
0x0000ffff). This can be useful for tc's bpf realm matcher, but
also a full hex/mask param can be provided already for matching
through iptables' --realm cmdline option, for example.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>