core: downgrade errors about BPF loading when called from socket_bind_supported()

core: downgrade errors about BPF loading when called from socket_bind_supported()

prepare_socket_bind_bpf() is called from two sites: socket_bind_supported() and
socket_bind_install_impl(). For the latter, when errors occur we certainly want
to log, since they'll be fatal for the unit.  But for the former, we should be
quiet, at least on the "expected" errors like lack of permissions. I kept error
on map resizing and such, which should not fail, at log_warning(). They are not
fatal when called from socket_bind_suppported(), but still a sign that
something is off.

Currently BPF filters can only be used by privileged users. Thus each systemd
--user will fail in socket_bind_supported(). With the patch, we only log this
at debug level.

https://lwn.net/ml/bpf/cover.1620499942.git.yifeifz2@illinois.edu/ gives some
hope that unprivileged access will be possible, so let's keep the code trying.
We might get lucky and get support for filters in user mode without any changes
on our side.