LU-2450: Do not store and use unreferenced oap pointers

LU-2450: Do not store and use unreferenced oap pointers

osc_extent_finish looks for the last oap to see if it was a partial
write or not, and does it by storing a pointer to it, but by the time
we need the offsets from this OAP, it might have already been freed
because completion was called.
Change the code to store oap offset and byte count instead.

Change-Id: Id52743e701d9b090907eb42ff4648c589193f000
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/4835
Tested-by: Hudson
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>