linuxkpi: remove global atomic counter of the task allocations

Use thread_reap_barrier() to ensure that no threads are kept in the
zombies list which could have the linuxkpi task allocated.

Also fix order of initialization and teardown for current task
allocation hooks and resources. Register current task allocator after
zones are initialized. Deregister allocator before cycling over threads
and zeroing task pointer.

Reviewed by: hselasky, markj
Sponsored by: Mellanox Technologies/NVidia Networking
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D30468