mptcp: pm: fix ID 0 endp usage after multiple re-creations

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

mptcp: pm: fix ID 0 endp usage after multiple re-creations

'local_addr_used' and 'add_addr_accepted' are decremented for addresses

not related to the initial subflow (ID0), because the source and

destination addresses of the initial subflows are known from the

beginning: they don't count as "additional local address being used" or

"ADD_ADDR being accepted".

It is then required not to increment them when the entrypoint used by

the initial subflow is removed and re-added during a connection. Without

this modification, this entrypoint cannot be removed and re-added more

than once.