rcu: Cleanup, don't put ->completed into an int

It is true that currently only the low-order two bits are used, so
there should be no problem given modern machines and compilers, but
good hygiene and maintainability dictates use of an unsigned long
instead of an int.  This commit therefore makes this change.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>