net/socket: Break down __sys_setsockopt

Split __sys_setsockopt() into two functions by removing the core
logic into a sub-function (do_sock_setsockopt()). This will avoid
code duplication when doing the same operation in other callers, for
instance.

do_sock_setsockopt() will be called by io_uring setsockopt() command
operation in the following patch.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20231016134750.1381153-4-leitao@debian.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>