LU-13799 lov: Cache stripe offset calculation

LU-13799 lov: Cache stripe offset calculation

Calculating the page offset relative to the stripe (etc)
in a file is surprisingly expensive.  Because i/o has
already been split up to stripes by the cl_io code,
calculating the stripe each time is unnecessary.

We cache most of the values requiring calculation.

This improves AIO/DIO page submission significantly,
improving performance by a bit over 10%.

Also remove lpg_generation, which isn't doing anything
useful.  This suggests the possibility of removing
lov_page, but that's for another patch.

This patch reduces i/o time in ms/GiB by:
Write: 17 ms/GiB
Read: 22 ms/GiB

Totals:
Write: 119 ms/GiB
Read: 121 ms/GiB

mpirun -np 1  $IOR -w -r -t 64M -b 64G -o ./iorfile --posix.odirect

With previous patches in series:
write        7531 MiB/s
read         7179 MiB/s

Plus this patch:
write        8637 MiB/s
read         8488 MiB/s

Signed-off-by: Patrick Farrell <farr0186@gmail.com>
Change-Id: I89e994592853d0fe93a034bfe8bdfb459bdaf584
Reviewed-on: https://review.whamcloud.com/39445
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>