Merge branch 'mptcp-fix-fallback-related-races'

Merge branch 'mptcp-fix-fallback-related-races'

Matthieu Baerts says:

====================
mptcp: fix fallback-related races

This series contains 3 fixes somewhat related to various races we have
while handling fallback.

The root cause of the issues addressed here is that the check for
"we can fallback to tcp now" and the related action are not atomic. That
also applies to fallback due to MP_FAIL -- where the window race is even
wider.

Address the issue introducing an additional spinlock to bundle together
all the relevant events, as per patch 1 and 2. These fixes can be
backported up to v5.19 and v5.15.

Note that mptcp_disconnect() unconditionally clears the fallback status
(zeroing msk->flags) but don't touch the `allows_infinite_fallback`
flag. Such issue is addressed in patch 3, and can be backported up to
v5.17.
====================

Link: https://patch.msgid.link/20250714-net-mptcp-fallback-races-v1-0-391aff963322@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>