wait for outstanding shadow variables free requests in kpatch_exit

Unload of kpatch module (and kpatch_shadow_hash table) before
all shadow variables free requests are processed can lead to
kernel crash.

Add rcu_barrier() to kpatch_exit() to wait for all outstanding
RCU callbacks to complete.

Resolves #468 
Signed-off-by: Jan Stancek <jstancek@redhat.com>