rcu: Check the return value of zalloc_cpumask_var()

This commit checks the return value of the zalloc_cpumask_var() used for
allocating cpumask for rcu_nocb_mask.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>