net: add __sys_socket_file()

This works like __sys_socket(), except instead of allocating and
returning a socket fd, it just returns the file associated with the
socket. No fd is installed into the process file table.

This is similar to do_accept(), and allows io_uring to use this without
instantiating a file descriptor in the process file table.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Acked-by: David S. Miller <davem@davemloft.net>
Link: https://lore.kernel.org/r/20220412202240.234207-2-axboe@kernel.dk