Merge branch 'generic-zcopy_-functions'

Merge branch 'generic-zcopy_-functions'

Jonathan Lemon says:

====================
Generic zcopy_* functions

This is set of cleanup patches for zerocopy which are intended
to allow a introduction of a different zerocopy implementation.

The top level API will use the skb_zcopy_*() functions, while
the current TCP specific zerocopy ends up using msg_zerocopy_*()
calls.

There should be no functional changes from these patches.
====================

Link: https://lore.kernel.org/r/20210106221841.1880536-1-jonathan.lemon@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>