rcu: Fix wrong check in list_splice_init_rcu()

If the list to be spliced is empty, then list_splice_init_rcu() has
nothing to do.  Unfortunately, list_splice_init_rcu() does not check
the list to be spliced; it instead checks the list to be spliced into.
This results in memory leaks given current usage.  This commit
therefore fixes the empty-list check.

Signed-off-by: Jan H. Schönherr <schnhrr@cs.tu-berlin.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>