libsystemd-network: fix endianness in ARP BPF filter (#8255)

Commit f11cba74 ("libsystemd-network: fix unaligned loads (issue #7654)")
changed the way in which the MAC address is read to use native endiannes:

 htobe32(*((uint32_t *)x)  ->  unaligned_read_ne32(x)

This is wrong because loads done with BPF_LD + BPF_ABS are big-endian, as it
can be seen for the ethertype and arp-operation loads above in the
filter.  Also, the same commit changed:

 htobe32(*((unsigned int *)x) -> unaligned_read_be32(x)

in _bind_raw_socket(), which is the correct form.

The commit broke IPv4LL in presence of loops, as the sender now considers its
own packets as conflicting.

Fixes: f11cba74