systemd: allow systemd_notify_t to send data to kernel_t datagram sockets

[rmetrich]

This is required because of systemd's notify socket is created while in the initramfs, hence as kernel_t. Once SELinux permits relabeling socket objects created before the policy is loaded, this should be removed and systemd fixed to relabel the socket appropriately. Tracked by systemd PR.

[zpytela]

Can you please include reproducer and/or the avc denials into the commit message?

[zpytela]

Merging, thank you.