rcu: Set rdp->gpwrap when CPU is idle

Commit #e3663b10 ("rcu: Handle gpnum/completed wrap while dyntick
idle") sets rdp->gpwrap on the wrong side of the "if" statement in
dyntick_save_progress_counter(), that is, it sets it when the CPU is
not idle instead of when it is idle.  Of course, if the CPU is not idle,
its rdp->gpnum won't be lagging beind the global rsp->gpnum, which means
that rdp->gpwrap will never be set.

This commit therefore moves this code to the proper leg of that "if"
statement.  This change means that the "else" cause is just "return 0"
and the "then" clause ends with "return 1", so also move the "return 0"
to follow the "if", dropping the "else" clause.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>