- Add md_mtx lock to protect ID number and list of devices. - Always check mdnew() return value, as even in !autounit case   kthread_create() can fail.
- Add md_mtx lock to protect ID number and list of devices.
- Always check mdnew() return value, as even in !autounit case
  kthread_create() can fail.

Those two changes fix serval panics provked by simple stress test.

Tested by:	Kris The BugMagnet
MFC after:	3 days

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