LU-1666 obdclass: reduce lock contention on coh_page_guard

LU-1666 obdclass: reduce lock contention on coh_page_guard

Define a per-page spinlock to get and put a cl_page instead of
grabbing per-object lock coh_page_guard.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Signed-off-by: Gregoire Pichon <gregoire.pichon@bull.net>
Change-Id: I3c63f0e33f3d4d44fbb5de06bcd0670e76b872bd
Reviewed-on: http://review.whamcloud.com/5428
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>