libafs: reset global icl set pointers on shutdown

Avoid panicking when an icl tracing function is called after
shutdown_icl.

There is a window during shutdown in which pioctls can be requested
after the shutdown_icl is issued. Reset the global icl set pointers
so tracing is disabled after the shutdown_icl, instead of using
pointers to freed memory.

Removed the unneeded afs_icl_FindSet calls and use the global
pointers which were set during the initialization.

Change-Id: I3310868a28850236a2870b8dab858ecb7a815c11
Reviewed-on: http://gerrit.openafs.org/10884
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>