io_uring/opdef: rename SENDZC_NOTIF to SEND_ZC

It's confusing to see the string SENDZC_NOTIF in ftrace output
when using IORING_OP_SEND_ZC.

Fixes: b48c312b

 ("io_uring/net: simplify zerocopy send user API")
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: io-uring@vger.kernel.org
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8e5cd8616919c92b6c3c7b6ea419fdffd5b97f3c.1663363798.git.metze@samba.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>