cxl/region: Fix memregion leaks in devm_cxl_add_region()

Move the mode verification to __create_region() before allocating the
memregion to avoid the memregion leaks.

Fixes: 6e099264

 ("cxl/region: Add volatile region creation support")
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/20240507053421.456439-1-lizhijian@fujitsu.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>