sbin/hammer: Don't show FLAG_BADMIRRORTID for root node in certain corner case situation

sbin/hammer: Don't show FLAG_BADMIRRORTID for root node in certain corner case situation

- Make hammer show command stop showwing an error flag regarding
  mirror tid that isn't actually an error.

- newfs_hammer initializes the root node's mirror_tid with 0 along
  with its first two elements, but this mirror_tid is not updated
  till the third element is allocated (by creating a file, etc).
  Following result shows this situation.

- In this situation the root node's node->mirror_tid (==0) being
  smaller than elm->base.create_tid or elm->base.delete_tid is not
  an error. This only happens on the root node because all the other
  nodes get updated when they split from existing ones.

     NODE 8000000020800000 cnt=02 p=0000000000000000 type=L depth=0 mirror 0000000000000000 fill=z8:65=1% {
B-----M	ELM  0 R lo=00000001 obj=0000000000000001 rt=01 key=0000000000000000 ot=01
	         tids 0000000100000001:0000000000000000
		 <snip>
B-----M	ELM  1 R lo=00000002 obj=0000000000000001 rt=15 key=0000000000000000 ot=00
	         tids 0000000100000001:0000000000000000
		 <snip>
     }