Add support for custom iov_iter handling to msghdr. The idea is that
in-kernel subsystems want control over how an SG is split.
Signed-off-by: NDavid Ahern <dsahern@kernel.org>
[pavel: move callback into msghdr]
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>