ceph_osd.cc: fix unreachable flush call

ceph_osd.cc: fix unreachable flush call

The calling chain for generic_server_usage():

  generic_server_usage()
    ->generic_usage(true)
    ->exit(1)
  cout.flush()

any statements after generic_server_usage() would not be reached,so we
need to flush cout in generic_usage().

Signed-off-by: Jiaying Ren <jiaying.ren@umcloud.com>