Missing writer side af_unix annotations in 4.19.317

[syphyr <syphyr@xxxxxxxxx>]

Several commits were made in v4.19.317 for af_unix to fix race 
conditions. Most of the READ_ONCE commits were added to v4.19.317, but 
the WRITE_ONCE on the writer side did not get added. For example, 
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=v5.15.163&id=5b9668fd874144d02888e55bb95ed5f4aacdf703