io_uring/napi: check napi_enabled in io_napi_add() before proceeding

doing so avoids the overhead of adding napi ids to all the rings that do
not enable napi.

if no id is added to napi_list because napi is disabled,
__io_napi_busy_loop() will not be called.

Signed-off-by: Olivier Langlois <olivier@trillion01.com>
Fixes: b4ccc4dd ("io_uring/napi: enable even with a timeout of 0")
Link: https://lore.kernel.org/r/bd989ccef5fda14f5fd9888faf4fefcf66bd0369.1723400131.git.olivier@trillion01.com


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