EX-7601 ofd: distinguish nr_write and nr_read

EX-7601 ofd: distinguish nr_write and nr_read

We will have two counts of pages in lnbs, distinguish
between them.

Not actually used yet - will be calculated when the read
lnb mapping is created.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I709b8fd299163d348a196184152bb0294fcb650b
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52985
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>