nvdimm-btt: simplify code with the scope based resource management

Use the scope based resource management (defined in
linux/cleanup.h) to automate resource lifetime
control on struct btt_sb *super in discover_arenas().

Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/20231214083919.22218-1-dinghao.liu@zju.edu.cn


Signed-off-by: Ira Weiny <ira.weiny@intel.com>