allocate_flower_entry: should check for null deref

allocate_flower_entry does not check for allocation success, but tries
to deref the result. I only moved the spin_lock under null check, because
 the caller is checking allocation's status at line 652.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>