With systemd the /run or /var/run are tmpfs filesystems. This means
the direcories required by samba need to be created during startup.
This can be done with a config file for tmpfiles, see samba.conf.tmp.
You need to copy this file to the directoy systemd-tmpfiles is looking
for its config files e.g.:

install -d -m 0755 /etc/tmpfiles.d/
install -m644 samba.conf.tmp /etc/tmpfiles.d/samba.conf

If you're a packager don't forget to run the systemd-tmpfiles binary
in the script after samba has been installed. This makes sure the
directory exists and you can start samba directly after the installation.

/usr/bin/systemd-tmpfiles --create /etc/tmpfiles.d/samba.conf
