LU-13131 osc: Make sure we don't accidentally deprioritize extents

LU-13131 osc: Make sure we don't accidentally deprioritize extents

Looks like in some cases high priority extents could migrate off
oo_hp_exts list back onto urgent list.

Change-Id: I96bf0a3a005b166f34dba215463c0806dfe2526a
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/37967
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>