Fix common misspellings

Fixes generated by 'codespell' and manually reviewed.

Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
diff --git a/include/net/transp_v6.h b/include/net/transp_v6.h
index eeb077d..a8122dc 100644
--- a/include/net/transp_v6.h
+++ b/include/net/transp_v6.h
@@ -16,7 +16,7 @@
 
 struct flowi6;
 
-/* extention headers */
+/* extension headers */
 extern int				ipv6_exthdrs_init(void);
 extern void				ipv6_exthdrs_exit(void);
 extern int				ipv6_frag_init(void);