LU-3384 llite: use READ, WRITE around ll_rw_stats_tally()

LU-3384 llite: use READ, WRITE around ll_rw_stats_tally()

In vvp_io_write_start() the stats function ll_rw_stats_tally() was
incorrectly called with a rw argument of 0. Correct this and use the
macros READ and WRITE in and around ll_rw_stats_tally() for clarity.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Icca6cfcc102b2944761ff211f1b2c2734aae8ecf
Reviewed-on: http://review.whamcloud.com/6447
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>