syscons - Avoid potential blocking issue.

syscons - Avoid potential blocking issue.

- kbd_ioctl() uses it's own locking via lockmgr() and the
  syscons softc is protected using a mutex, so we better
  avoid both locks at once due possible blocking issues.
  In fact, in my particular case the panic only occurs in
  real hardware, while on a VMWare VM I had a lockup.
- Only found in 4-CPU SMP setups.
- Discovered via DEBUG_LOCKS option.

DragonFly-bug: <http://bugs.dragonflybsd.org/issue2148>
Reviewed-by: @dillon, @sjg