sparc64: Fix irq stack bootmem allocation.

sparc64: Fix irq stack bootmem allocation.

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.

Orabug: 23050718

Signed-off-by: Atish Patra <atish.patra@oracle.com>
Reviewed-by: Bob Picco <bob.picco@oracle.com>
Reviewed-by: Vijay Kumar <vijay.ac.kumar@oracle.com>
(cherry picked from commit d192d0743aa6dbca0991900c490c06111b4bd86c)
Signed-off-by: Allen Pais <allen.pais@oracle.com>