LU-3416 llite: reset writeback index in ll_writepages

LU-3416 llite: reset writeback index in ll_writepages

Otherwise after one round the writeback index will become beyond
the file size and ->writepages() turns into an empty operation.

Also, a safety guard is added to limit the wait time for grant to
10 minutes(take recovery into consideration) at maximum in the
osc_enter_cache() function. Otherwise, EDQUOT will be returned to
the applications to start sync write.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I7d0b90ef0c15f069ec63435703622a327b75ad70
Reviewed-on: http://review.whamcloud.com/7657
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Tested-by: Hudson
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>