init-radosgw.sysv: set ulimit -n before starting daemon

If we do the ulimit inside the daemon command we will have already
dropped privs and will fail.

Fixes: #9587
Backport: giant, firefly
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2772 from ceph/wip-init-radosgw

init-radosgw.sysv: set ulimit -n before starting daemon

Reviewed-by: Loic Dachary <ldachary@redhat.com>