RE: [Patch net-next v3] net: clean up skb headers code

commit 1a37e412a0225fcba5587 (net: Use 16bits for *_headers
fields of struct skbuff) converts skb->*_header to u16,
some #if NET_SKBUFF_DATA_USES_OFFSET are now useless,
and to be safe, we could just use "X = ~(typeof(X))0;"
as suggested by David and Ben.

IIRC ~(unsigned short)0 is the same as ~0 and will be -1.
define an actual constant with value 0xffff

	David