staging: lustre: 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>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-2779
Reviewed-on: http://review.whamcloud.com/5419


Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>