rcutorture: Make consistent use of variables

The "if" statement at the beginning of rcu_torture_writer() should
use the same set of variables.  In theory, this does not matter because
the corresponding variables (gp_sync and gp_sync1) have the same value
at this point in the code, but in practice such puzzles should be
removed.  This commit therefore makes the use of variables consistent.

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