crtools/usage: error on excessive subcommands

Before:
  [~]# criu check lazy_pages
  Looks good.
  [~]# criu check --feature lazy_pages
  Error (criu/kerndat.c:588): Lazy pages are not available
After:
  [~]# criu check lazy_pages
  Error: excessive parameters for command check
  ...skipped usage() output...

Just a little improvement, not to mislead anyone.

Cc: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Reviewed-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>