if_gre: Use OID_AUTO for the net.link.gre sysctl nodeHEADmain

if_gre(4) registers net.link.gre with the static OID number IFT_TUNNEL.
That number is an interface type that several tunnel drivers share, not
one that belongs to gre(4).  if_me(4) used it too, which made loading
both modules panic until 21f2245de219.  Nothing addresses the node by
number, so use OID_AUTO as if_me(4) now does.

Suggested by:	markj
MFC after:	3 days
Sponsored by:	Rubicon Communications, LLC ("Netgate")

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