sysctl: do not fail systemd-sysctl.service if /proc/sys is mounted read-only

sysctl: do not fail systemd-sysctl.service if /proc/sys is mounted read-only

Let's make missing write access to /proc/sys non-fatal to the sysctl service.

This is a follow-up to 411e869f497c7c7bd0688f1e3500f9043bc56e48 which altered
the condition for running the sysctl service to check for /proc/sys/net being
writable, accepting that /proc/sys might be read-only. In order to ensure the
boot-up stays clean in containers lower the log level for the EROFS errors
generated due to this.