networkd: Don't stop networkd if CONFIG_FIB_RULES=n in kernel (#7030)

If FIB Rules are not supported by the kernel then networkd fails to
start as it retuns error=-EOPNOTSUPP.
In this case just ignore and let start networkd.

```
sendto(5, {{len=28, type=RTM_GETRULE, flags=NLM_F_REQUEST|NLM_F_ACK|NLM_F_DUMP, seq=8, pid=0}, {family=AF_UNSPEC, dst_len=0, src_len=0, tos=0, table=RT_TABLE_UNSPEC, action=FR_ACT_TO_TBL, flags=0}}, 28, 0, {sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, 16) = 28
recvmsg(5, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=128->12, msg_iov=[{iov_base=NULL, iov_len=0}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_NETLINK, cmsg_type=0x3}], msg_controllen=24, msg_flags=MSG_TRUNC}, MSG_PEEK|MSG_TRUNC) = 48
recvmsg(5, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=128->12, msg_iov=[{iov_base={{len=48, type=NLMSG_ERROR, flags=0, seq=8, pid=8856}, {error=-EOPNOTSUPP, msg={{len=28, type=RTM_GETRULE, flags=NLM_F_REQUEST|NLM_F_ACK|NLM_F_DUMP, seq=8, pid=0}, {family=AF_UNSPEC, dst_len=0, src_len=0, tos=0, table=RT_TABLE_UNSPEC, action=FR_ACT_TO_TBL, flags=0}}}}, iov_len=7416}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_NETLINK, cmsg_type=0x3}], msg_controllen=24, msg_flags=0}, MSG_TRUNC) = 48
```

```
Oct 08 10:22:24 naomi systemd[1]: Starting Network Service...
Oct 08 10:22:24 naomi systemd-networkd[983]: Could not enumerate rules: Operation not supported
Oct 08 10:22:24 naomi systemd[1]: systemd-networkd.service: Main process exited, code=exited, status=1/FAILURE
```

Fixes #7027

(cherry picked from commit 6acbbdd4)