Currently, irq stack bootmem is allocated for all possible cpus
before nr_cpus value changes the list of possible cpus. As a result,
there is unnecessary wastage of bootmemory.

Move the irq stack bootmem allocation so that it happens after
possible cpu list is modified based on nr_cpus value.
Signed-off-by: NAtish Patra <atish.patra@oracle.com>
Reviewed-by: NBob Picco <bob.picco@oracle.com>
Reviewed-by: NVijay Kumar <vijay.ac.kumar@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>