MRP-1071 lprocfs: fix crash during opening "per_cpu_rebuild"

Opening "per_cpu_rebuild" from lprocfs in mc on a freshly mounted
Lustre fs causes kernel crash.

The NULL pointer deference is caused by, ret = lc->lc_count in
lprocfs_read_helper. The *lc in this, is assigned by
lprocfs_rd_percpu_rebuild.
In this change, the call to lprocfs_read_helper is included
within lprocfs_stats_lock/unlock semantics.
stat info is allocated in lprocfs_stats_lock(LPROCFS_GET_NUM_CPU).
Call to this, would help get the correct stats w/o causing any panic

Signed-off-by: Parinay Kondekar <Parinay_Kondekar@xyratex.com>
Signed-off-by: Alexander Boyko <alexander_boyko@xyratex.com>
Change-Id: Ib2ff6b037d5d7fd5375f827259551f99f5587a67
Reviewed-on: http://morpheus.xyus.xyratex.com:8443/gerrit/477
Tested-by: Jenkins
Reviewed-by: Andrew Perepechko <Andrew_Perepechko@xyratex.com>
Reviewed-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Tested-by: Elena Gryaznova <Elena_Gryaznova@xyratex.com>
Reviewed-by: Denis Kondratenko <Denis_Kondratenko@xyratex.com>