It applies cleanly on my 2.0 checkout tree.

[tahngarth] /Development/DragonFly-2.0/src/sys/net> patch -C <
/tmp/ethersubr.txt
Hmm...  Looks like a unified diff to me...
The text leading up to this was:
--------------------------
|Index: if_ethersubr.c
|===================================================================
|RCS file: /dcvs/src/sys/net/if_ethersubr.c,v
|retrieving revision 1.76
|diff -u -p -r1.76 if_ethersubr.c
|--- if_ethersubr.c     8 Jul 2008 13:50:52 -0000       1.76
|+++ if_ethersubr.c     24 Jul 2008 01:56:50 -0000
--------------------------
Patching file if_ethersubr.c using Plan A...
Hunk #1 succeeded at 1658.
Hunk #2 succeeded at 1668.
done