This adds some minor functionality to the per_task API for use by the
upcoming offline waiting work.
Add scoutfs_per_task_add_excl() so that a caller can tell if their task
was already put on a per-task list by their caller.
Make scoutfs_per_task_del() return a bool to indicate if the entry was
found on a list and was in fact deleted, or not.
Add scoutfs_per_task_init_entry() for initializing entries that aren't
declared on the stack.
Signed-off-by: Zach Brown <zab@versity.com>