io_uring/net: Support multishot receive len cap

io_uring/net: Support multishot receive len cap

At the moment its very hard to do fine grained backpressure when using
multishot as the kernel might produce a lot of completions before the
user has a chance to cancel a previous submitted multishot recv.

This change adds support to issue a multishot recv that is capped by a
len, which means the kernel will only rearm until X amount of data is
received. When the limit is reached the completion will signal to the
user that a re-arm needs to happen manually by not setting the IORING_CQE_F_MORE
flag.

Signed-off-by: Norman Maurer <norman_maurer@apple.com>
Link: https://lore.kernel.org/r/20250715140249.31186-1-norman_maurer@apple.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>