mptcp: pm: in-kernel: increase all limits to 64

This means switching the maximum from 8 to 64 for the number of subflows
and accepted ADD_ADDR.

The previous limit of 8 subflows makes sense in most cases. Using more
subflows will very likely *not* improve the situation, and could even
decrease the performances. But there are no technical limitations nor
performance impact to raise this limit, so let's do it: this will allow
people with very specific use-cases, and researchers to easily create
more subflows, and measure the performance impact by themselves.

The theoretical limit is 255 -- the ID is written in a u8 on the wire --
but 64 is more than enough. With so many subflows, it will be costly to
iterate over all of them when operations are done in bottom half.

Note that the in-kernel PM will continue to create subflows in reply to
ADD_ADDR with a single batch of maximum 8 subflows. Same when adding new
"subflow" endpoints with the fullmesh flag. Increasing those batch
limits would have a memory impact, and it looks fine not to cover these
cases with larger batches for the moment. If more is needed later, the
position of the last subflow from the list could be remembered, and the
list iteration could continue later.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/434
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260508-net-next-mptcp-pm-inc-limits-v1-2-c84e3fdf9b6a@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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