LU-3055 libcfs: remove page_collection::pc_lock

LU-3055 libcfs: remove page_collection::pc_lock

page_collection::pc_lock is supposed to protect race between
functions called by smp_call_function(), however we don't have
this use-case for ages and page_collection only lives in stack
of thread, so it is safe to remove it.

Signed-off-by: Liang Zhen <liang.zhen@intel.com>
Change-Id: I89ca46e6593838d85fc0b91d1ff7c35cc28cf4ce
Reviewed-on: http://review.whamcloud.com/7660
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Sebastien Buisson <sebastien.buisson@bull.net>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>