LU-3035 clio: improper LASSERT in ll_file_io_generic()

LU-3035 clio: improper LASSERT in ll_file_io_generic()

This LASSERT was introduced from the fix of LU-2910, and
which is incorrect, since the crw_count could be changed
in lov_io_rw_iter_init() even if no any read/write done.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: Ib2cee88f0a7f75f8fb63330912aa053fb5b9393e
Reviewed-on: http://review.whamcloud.com/5864
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Hudson
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>