uaccess: add copy_struct_{from,to}_bounce_buffer() helpers

uaccess: add copy_struct_{from,to}_bounce_buffer() helpers

These are similar to copy_struct_{from,to}_user() but operate
on kernel buffers instead of user buffers.

They can be used when there is a temporary bounce buffer used,
e.g. in msg_control or similar places.

It allows us to have the same logic to handle old vs. current
and current vs. new structures in the same compatible way.

copy_struct_from_sockptr() will also be able to
use copy_struct_from_bounce_buffer() for the kernel
case as follow us patch.

I'll use this in my IPPROTO_SMBDIRECT work,
but maybe it will also be useful for others...
IPPROTO_QUIC will likely also use it.

Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: Dmitry Safonov <dima@arista.com>
Cc: Francesco Ruggeri <fruggeri@arista.com>
Cc: Salam Noureddine <noureddine@arista.com>
Cc: David Ahern <dsahern@kernel.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Michal Luczaj <mhal@rbox.co>
Cc: David Wei <dw@davidwei.uk>
Cc: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Cc: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Xin Long <lucien.xin@gmail.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Simon Horman <horms@kernel.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Christian Brauner <brauner@kernel.org>
CC: Kees Cook <keescook@chromium.org>
Cc: netdev@vger.kernel.org
Cc: linux-bluetooth@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Link: https://patch.msgid.link/f29570914590c50b9b6f451eb3a38d0fe1d954df.1775576651.git.metze@samba.org
Signed-off-by: Christian Brauner <brauner@kernel.org>