AZL-88824

In the Linux kernel, the following vulnerability has been resolved:

mptcp: pm: ADD_ADDR rtx: always decrease sk refcount

When an ADDADDR is retransmitted, the sk is held in skreset_timer(). It should then be released in all cases at the end.

Some (unlikely) checks were returning directly instead of calling sock_put() to decrease the refcount. Jump to a new 'exit' label to call __sockput() (which will become sockput() in the next commit) to fix this potential leak.

While at it, drop the '!msk' check which cannot happen because it is never reset, and explicitly mark the remaining one as "unlikely".