LU-9679 osc: discard oe_intree

LU-9679 osc: discard oe_intree

An rbnode knows if it is in the tree or not, using RB_EMPTY_NODE().
There is no need for an extra flag.

Linux-Commit: a25a82301800 ("lustre: osc_cache: discard oe_intree")

Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Mr NeilBrown <neilb@suse.com>
Change-Id: Iddd4ef623d96c1cf2e0789343c02ecb095f7b6a1
Reviewed-on: https://review.whamcloud.com/37600
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>