io_uring/napi: improve __io_napi_add

1. move the sock->sk pointer validity test outside the function to
   avoid the function call overhead and to make the function more
   more reusable
2. change its name to __io_napi_add_id to be more precise about it is
   doing
3. return an error code to report errors

Signed-off-by: Olivier Langlois <olivier@trillion01.com>
Link: https://lore.kernel.org/r/d637fa3b437d753c0f4e44ff6a7b5bf2c2611270.1728828877.git.olivier@trillion01.com


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