seccomp: ensure rules are loaded in seccomp_memory_deny_write_execute

If seccomp_memory_deny_write_execute was fatally failing to load rules it
already returned a bad retval.
But if any adding filters failed it skipped the subsequent seccomp_load and
always returned an rc of 0 even if no rule was loaded at all.

Lets fix this requiring to (non fatally-failing) load at least one rule set.

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>