LU-8066 osc: restore cur_dirty_grant_bytes

LU-8066 osc: restore cur_dirty_grant_bytes

A previous patch which moved some attrs from /proc to /sys
inadvertently removed 'cur_dirty_grant_bytes'.
This patch restores it in /sys.

Test-Parameters: trivial
Fixes: 32fb31f3bf3d ("LU-8066 osc: move suitable values from procfs to sysfs")
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ic6cff067d3634d1b1406dad4eb156ad1967042c9
Reviewed-on: https://review.whamcloud.com/39740
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>