kernel - Fix kernel panic caused by rename race

* kern_rename() must temporarily unlock the source ncp when resolving the
  target ncp, and will sometimes have to temporarily unlock the target ncp
  when relocking both.  This can race against a topological removal of one
  or both ncp's, resulting in incorrect operation or a panic due to a
  NULL vp.

* Detect the case and return EAGAIN if the ncp's have been marked NCF_DESTROYED
  or if fromncp loses its vp resolution.  Callers detect the EAGAIN and retry
  the operation.

Reported-by: ftigeot