PERCPU : __percpu_alloc_mask() can dynamically size percpu_data storage

Instead of allocating a fix sized array of NR_CPUS pointers for percpu_data,
we can use nr_cpu_ids, which is generally < NR_CPUS.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>