LU-1282 misc: Use present cpu numbers to save memory.

lprocfs stats data should allocated by the number of present cpus in
stead of by possible cpu number which wastes a lot of memory.

When new cpus are hot-plugged in, alloc necessary percpu array
elements on demand.

OSS minimum thread number also better be decided by online cpu
number.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: Id1690f185f4f83fae75be7eddb756e413cbc4fba
Reviewed-on: http://review.whamcloud.com/2451
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>