rcu: Don't allocate bootmem from rcu_init()

When rcu_init() is called we already have slab working, allocating
bootmem at that point results in warnings and an allocation from
slab.  This commit therefore changes alloc_bootmem_cpumask_var() to
alloc_cpumask_var() in rcu_bootup_announce_oddness(), which is called
from rcu_init().

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Tested-by: Robin Holt <holt@sgi.com>

[paulmck: convert to zalloc_cpumask_var(), as suggested by Yinghai Lu.]