sysctl.d: bump number of queueable AF_UNIX/SOCK_DGRAM datagrams

The default of 16 is pretty low, let's bump this to accomodate for more
queued datagrams. This is useful for AF_UNIX/SOCK_DGRAM logging and
sd_notify() sockets as this allows queuing more datagrams before things
start to block, thus improving parallelization and logging performance.

sysctl.d/50-default.conf

@@ -35,6 +35,9 @@ net.ipv4.conf.all.promote_secondaries = 1
 # Fair Queue CoDel packet scheduler to fight bufferbloat
 net.core.default_qdisc = fq_codel
 
+# Make sure we can queue more than just a few datagrams in AF_UNIX sockets.
+net.unix.max_dgram_qlen = 512
+
 # Enable hard and soft link protection
 fs.protected_hardlinks = 1
 fs.protected_symlinks = 1