Implement proper support for complete_all() in the LinuxKPI.

When complete_all() is called there might be multiple waiters. The
current implementation could only handle one waiter. Make sure the
completion is sticky when complete_all() is called to be compatible
with Linux.

Found by: Johannes Lundberg <johalun0@gmail.com>
MFC after: 1 week
Sponsored by: Mellanox Technologies
Sponsored by: Limelight Networks