net: remove {revc,send}msg_copy_msghdr() from exports

The only user of these was io_uring, and it's not using them anymore.
Make them static and remove them from the socket header file.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/1b6089d3-c1cf-464a-abd3-b0f0b6bb2523@kernel.dk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>