core/bpf-firewall: make bpf_firewall_supported() always set unsupported reason...
core/bpf-firewall: make bpf_firewall_supported() always set unsupported reason when BPF_FIREWALL_UNSUPPORTED is returned
Otherwise, log_unit_full_errno() in emit_bpf_firewall_warning() will
trigger an assertion.
(cherry picked from commit 8751bb6f)
(cherry picked from commit 27028ef0)