dm persistent data: remove unused return from exit_shadow_spine()

Fix the following coccicheck warnings:

./drivers/md/persistent-data/dm-btree-spine.c:188:5-6: Unneeded
variable: "r". Return "0" on line 194.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>