mptcp: fix NULL pointer in can_accept_new_subflow

...

UBUNTU-CVE-2025-23145

In the Linux kernel, the following vulnerability has been resolved: mptcp: fix NULL pointer in canacceptnewsubflow When testing valkey benchmark tool with MPTCP, the kernel panics in 'mptcpcanacceptnewsubflow' because subflowreq-msk is NULL. Call trace: mptcpcanacceptnewsubflow...