io_uring/net: harden multishot termination case for recv

If the recv returns zero, or an error, then it doesn't matter if more
data has already been received for this buffer. A condition like that
should terminate the multishot receive. Rather than pass in the
collected return value, pass in whether to terminate or keep the recv
going separately.

Note that this isn't a bug right now, as the only way to get there is
via setting MSG_WAITALL with multishot receive. And if an application
does that, then -EINVAL is returned anyway. But it seems like an easy
bug to introduce, so let's make it a bit more explicit.

Link: https://github.com/axboe/liburing/issues/1246


Cc: stable@vger.kernel.org
Fixes: b3fdea6e ("io_uring: multishot recv")
Signed-off-by: Jens Axboe <axboe@kernel.dk>