Revert "LU-2779 osc: Do not assert writeback in discard_cb()"

This reverts commit 571db9ac4826fbc900a95992fcfc6e35a4b1d463.
LU-2779 osc: osc_extent_wait() shouldn't be interruptible

Otherwise it will hit the assertion at cl_lock.c:

   cl_lock.c:1967:discard_cb())
	ASSERTION( (!(page->cp_type == CPT_CACHEABLE) ||
	(!PageWriteback(cl_page_vmpage(env, page)))) ) failed:

This is because in osc_lock_flush() we have to make sure the IO
is finished before discarding the pages.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I296ab35d26b861f3c57ddba9f74564aa439aaa0b
Reviewed-on: http://review.whamcloud.com/5419
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>