sys/vfs/hammer: Keep root node signature on root split

sys/vfs/hammer: Keep root node signature on root split

- newfs_hammer sets a signature value for the root btree node
  so keep that signature on root node split along with other
  fields being copied to the new root node. This doesn't affect
  the existing fs as it's used for nothing.

- (The former root node signature is not cleared on root split,
  so every node that has once been the root has the signature.
  The one at the lowest zone offset is the original one unless
  moved by reblock)