MINOR: list: adding MT_LIST_APPEND_LOCKED macro

MINOR: list: adding MT_LIST_APPEND_LOCKED macro

adding a new mt macro: MT_LIST_APPEND_LOCKED.

This macro may be used to append an item to an existing
list, like MT_LIST_APPEND.

But here the item will be forced into locked/busy state
prior to appending, so that it is already referenced
in the list while still preventing concurrent accesses
until we decide to unlock it.

The macro returns a struct mt_list "np", that is needed
at unlock time using regular MT_LIST_UNLOCK_ELT macro.